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Schedule III (Details)
 12 Months Ended
Dec. 31, 2015
USD ($)
apartment
Dec. 31, 2014
USD ($)
Dec. 31, 2013
USD ($)
Dec. 31, 2015
ft²
Dec. 31, 2015
USD ($)
Dec. 31, 2015
unit
Jun. 30, 2015
ft²
Dec. 31, 2014
USD ($)
ft²
Feb. 21, 2014
unit
Dec. 31, 2013
USD ($)
ft²
Oct. 02, 2013
unit
Real Estate and Accumulated Depreciation [Line Items]                      
Initial Cost, Land [1]         $ 588,663,000            
Initial Cost, Buildings and Improvements [1]         1,448,929,000            
Net Improvements (Retirement) since Acquisition         636,299,000            
Gross Amounts at Which Carried at December 31, 2011, Land         561,256,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         2,112,635,000            
Gross Amounts at Which Carried at December 31, 2011, Total $ 2,547,188,000 $ 2,289,509,000 $ 2,529,131,000   2,673,891,000 [2]     $ 2,547,188,000   $ 2,289,509,000  
Accumulated Depreciation at December 31, 2011 $ 640,434,000 611,408,000 610,536,000   692,608,000     640,434,000   $ 611,408,000  
Net Rentable Square Feet | ft² [3]       10,212,000              
Units 506         3,258          
Mortgage notes payable         418,052,000     $ 417,194,000      
Real estate, federal income tax basis         2,071,600,000            
Area of land | ft²       197,000       430,000   1,172,000  
Development in Process         36,094,000     $ 76,235,000      
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, beginning of period $ 2,547,188,000 2,289,509,000 2,529,131,000                
Property acquisitions [4] 162,702,000 289,140,000 47,444,000                
Improvements [4] 50,954,000 98,250,000 71,127,000                
Impairment write-down (5,909,000) 0 0                
Write-off of disposed assets (3,291,000) (2,857,000) (2,017,000)                
Property sales (77,753,000) (126,854,000) (356,176,000)                
Balance, end of period 2,673,891,000 [2] 2,547,188,000 2,289,509,000                
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, beginning of period 640,434,000 611,408,000 610,536,000                
Depreciation 86,536,000 77,741,000 80,510,000                
Impairment write-down 0 0 0                
Write-off of disposed assets (2,408,000) (2,549,000) (1,404,000)                
Property sales (31,954,000) (46,166,000) (78,234,000)                
Balance, end of period 692,608,000 $ 640,434,000 $ 611,408,000                
Multifamily Properties [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Initial Cost, Land [1]         125,089,000            
Initial Cost, Buildings and Improvements [1]         297,008,000            
Net Improvements (Retirement) since Acquisition         235,634,000            
Gross Amounts at Which Carried at December 31, 2011, Land         112,496,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         545,235,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] 657,731,000       657,731,000            
Accumulated Depreciation at December 31, 2011 141,200,000       141,200,000            
Net Rentable Square Feet | ft² [3]       3,014,000              
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] 657,731,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 141,200,000                    
3801 Connecticut [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location [5] Washington, DC                    
Initial Cost, Land [1],[5]         420,000            
Initial Cost, Buildings and Improvements [1],[5]         2,678,000            
Net Improvements (Retirement) since Acquisition [5]         12,961,000            
Gross Amounts at Which Carried at December 31, 2011, Land [5]         420,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements [5]         15,639,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2],[5] $ 16,059,000       16,059,000            
Accumulated Depreciation at December 31, 2011 [5] $ 9,901,000       9,901,000            
Year of Construction [5] Jan. 01, 1951                    
Date of Acquisition [5] Jan. 01, 1963                    
Net Rentable Square Feet | ft² [3]       178,000              
Units | unit           307          
Depreciation Life [6] 30 years                    
Mortgage notes payable         35,400,000            
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2],[5] $ 16,059,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period [5] $ 9,901,000                    
Roosevelt Towers [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Virginia                    
Initial Cost, Land [1]         336,000            
Initial Cost, Buildings and Improvements [1]         1,996,000            
Net Improvements (Retirement) since Acquisition         11,738,000            
Gross Amounts at Which Carried at December 31, 2011, Land         336,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         13,734,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 14,070,000       14,070,000            
Accumulated Depreciation at December 31, 2011 $ 8,886,000       8,886,000            
Year of Construction Jan. 01, 1964                    
Date of Acquisition May 01, 1965                    
Net Rentable Square Feet | ft² [3]       170,000              
Units | unit           191          
Depreciation Life [6] 40 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 14,070,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 8,886,000                    
Park Adams [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Virginia                    
Initial Cost, Land [1]         287,000            
Initial Cost, Buildings and Improvements [1]         1,654,000            
Net Improvements (Retirement) since Acquisition         11,270,000            
Gross Amounts at Which Carried at December 31, 2011, Land         287,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         12,924,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 13,211,000       13,211,000            
Accumulated Depreciation at December 31, 2011 $ 8,901,000       8,901,000            
Year of Construction Jan. 01, 1959                    
Date of Acquisition Jan. 01, 1969                    
Net Rentable Square Feet | ft² [3]       173,000              
Units | unit           200          
Depreciation Life [6] 35 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 13,211,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 8,901,000                    
The Ashby at McLean [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Virginia                    
Initial Cost, Land [1]         4,356,000            
Initial Cost, Buildings and Improvements [1]         17,102,000            
Net Improvements (Retirement) since Acquisition         17,835,000            
Gross Amounts at Which Carried at December 31, 2011, Land         4,356,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         34,937,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 39,293,000       39,293,000            
Accumulated Depreciation at December 31, 2011 $ 22,176,000       22,176,000            
Year of Construction Jan. 01, 1982                    
Date of Acquisition Aug. 01, 1996                    
Net Rentable Square Feet | ft² [3]       274,000              
Units | unit           256          
Depreciation Life [6] 30 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 39,293,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 22,176,000                    
Walker House Apartments [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location [5] Maryland                    
Initial Cost, Land [1],[5]         2,851,000            
Initial Cost, Buildings and Improvements [1],[5]         7,946,000            
Net Improvements (Retirement) since Acquisition [5]         7,042,000            
Gross Amounts at Which Carried at December 31, 2011, Land [5]         2,851,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements [5]         14,988,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2],[5] $ 17,839,000       17,839,000            
Accumulated Depreciation at December 31, 2011 [5] $ 10,197,000       10,197,000            
Year of Construction [5] Jan. 01, 1971                    
Date of Acquisition [5] Mar. 01, 1996                    
Net Rentable Square Feet | ft² [3]       157,000              
Units | unit           212          
Depreciation Life [6] 30 years                    
Mortgage notes payable         16,500,000            
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2],[5] $ 17,839,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period [5] $ 10,197,000                    
Bethesda Hill Apartments [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location [5] Maryland                    
Initial Cost, Land [1],[5]         3,900,000            
Initial Cost, Buildings and Improvements [1],[5]         13,412,000            
Net Improvements (Retirement) since Acquisition [5]         12,733,000            
Gross Amounts at Which Carried at December 31, 2011, Land [5]         3,900,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements [5]         26,145,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2],[5] $ 30,045,000       30,045,000            
Accumulated Depreciation at December 31, 2011 [5] $ 16,650,000       16,650,000            
Year of Construction [5] Jan. 01, 1986                    
Date of Acquisition [5] Nov. 01, 1997                    
Net Rentable Square Feet | ft² [3],[5]       225,000              
Units | unit           195          
Depreciation Life [6] 30 years                    
Mortgage notes payable         29,100,000            
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2],[5] $ 30,045,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period [5] $ 16,650,000                    
Bennett Park [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Virginia                    
Initial Cost, Land [1]         2,861,000            
Initial Cost, Buildings and Improvements [1]         917,000            
Net Improvements (Retirement) since Acquisition         80,036,000            
Gross Amounts at Which Carried at December 31, 2011, Land         4,774,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         79,040,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 83,814,000       83,814,000            
Accumulated Depreciation at December 31, 2011 $ 29,053,000       29,053,000            
Year of Construction Jan. 01, 2007                    
Date of Acquisition Feb. 01, 2001                    
Net Rentable Square Feet | ft² [3]       214,000              
Units | unit           224          
Depreciation Life [6] 28 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 83,814,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 29,053,000                    
The Clayborne [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Virginia                    
Initial Cost, Land [1]         269,000            
Initial Cost, Buildings and Improvements [1]         0            
Net Improvements (Retirement) since Acquisition         30,754,000            
Gross Amounts at Which Carried at December 31, 2011, Land         699,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         30,324,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 31,023,000       31,023,000            
Accumulated Depreciation at December 31, 2011 $ 12,616,000       12,616,000            
Year of Construction Jan. 01, 2008                    
Date of Acquisition Jun. 01, 2003                    
Net Rentable Square Feet | ft² [3]       60,000              
Units | unit           74          
Depreciation Life [6] 26 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 31,023,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 12,616,000                    
The Kenmore [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location [5] Washington, DC                    
Initial Cost, Land [1],[5]         28,222,000            
Initial Cost, Buildings and Improvements [1],[5]         33,955,000            
Net Improvements (Retirement) since Acquisition [5]         10,679,000            
Gross Amounts at Which Carried at December 31, 2011, Land [5]         28,222,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements [5]         44,634,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2],[5] $ 72,856,000       72,856,000            
Accumulated Depreciation at December 31, 2011 [5] $ 10,436,000       10,436,000            
Year of Construction [5] Jan. 01, 1948                    
Date of Acquisition [5] Sep. 01, 2008                    
Net Rentable Square Feet | ft² [3]       268,000              
Units | unit           374          
Depreciation Life [6] 30 years                    
Mortgage notes payable         33,600,000            
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2],[5] $ 72,856,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period [5] $ 10,436,000                    
Six Fifty North Glebe Road [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location [7] Virginia                    
Initial Cost, Land [1],[5]         12,787,000            
Initial Cost, Buildings and Improvements [1],[7]         0            
Net Improvements (Retirement) since Acquisition [7]         37,854,000            
Gross Amounts at Which Carried at December 31, 2011, Land [7]         12,851,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements [7]         37,790,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2],[7] $ 50,641,000       50,641,000            
Accumulated Depreciation at December 31, 2011 [7] $ 2,347,000       2,347,000            
Year of Construction [5] Jan. 01, 2014                    
Date of Acquisition [7] Jun. 01, 2011                    
Net Rentable Square Feet | ft² [3]       139,000              
Units | unit           163          
Depreciation Life [6] 30 years                    
Mortgage notes payable         32,214,000     27,690,000      
Development in Process         1,188,000     0      
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2],[7] $ 50,641,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period [7] $ 2,347,000                    
The Paramount [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Virginia                    
Initial Cost, Land [1]         8,568,000            
Initial Cost, Buildings and Improvements [1]         38,716,000            
Net Improvements (Retirement) since Acquisition         1,408,000            
Gross Amounts at Which Carried at December 31, 2011, Land         8,568,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         40,124,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 48,692,000       48,692,000            
Accumulated Depreciation at December 31, 2011 $ 3,894,000       3,894,000            
Year of Construction Jan. 01, 1984                    
Date of Acquisition [5] Oct. 01, 2013                    
Net Rentable Square Feet | ft² [3]       141,000              
Units | unit           135         135
Depreciation Life [6] 30 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 48,692,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 3,894,000                    
Yale West [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location [5] Washington, DC                    
Initial Cost, Land [1],[5]         14,684,000            
Initial Cost, Buildings and Improvements [1],[5]         62,069,000            
Net Improvements (Retirement) since Acquisition [5]         245,000            
Gross Amounts at Which Carried at December 31, 2011, Land [5]         14,684,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements [5]         62,314,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2],[5] $ 76,998,000       76,998,000            
Accumulated Depreciation at December 31, 2011 [5] $ 4,196,000       4,196,000            
Year of Construction [5] Jan. 01, 2011                    
Date of Acquisition [5] Feb. 01, 2014                    
Net Rentable Square Feet | ft² [3],[5]       173,000              
Units | unit           216     216    
Depreciation Life [6] 30 years                    
Mortgage notes payable         51,900,000            
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2],[5] $ 76,998,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period [5] $ 4,196,000                    
The Wellington [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location [5] Virginia                    
Initial Cost, Land [1]         30,548,000            
Initial Cost, Buildings and Improvements [1],[5]         116,563,000            
Net Improvements (Retirement) since Acquisition [5]         420,000            
Gross Amounts at Which Carried at December 31, 2011, Land [5]         30,548,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements [5]         116,983,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2],[5] $ 147,531,000       147,531,000            
Accumulated Depreciation at December 31, 2011 [5] $ 1,947,000       1,947,000            
Year of Construction [5] Jan. 01, 1960                    
Date of Acquisition [5] Jul. 01, 2015                    
Net Rentable Square Feet | ft² [3],[5]       842,000              
Units | unit           711          
Depreciation Life [6] 30 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2],[5] $ 147,531,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period [5] $ 1,947,000                    
The Wellington land parcel [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location [5] Virginia                    
Initial Cost, Buildings and Improvements [1],[5]         0            
Net Improvements (Retirement) since Acquisition [5]         659,000            
Gross Amounts at Which Carried at December 31, 2011, Land [5]         0            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements [5]         15,659,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2],[5] $ 15,659,000       15,659,000            
Accumulated Depreciation at December 31, 2011 [5] $ 0       0            
Date of Acquisition [5] Jul. 01, 2015                    
Development in Process         15,000,000 [1],[7]     $ 15,700,000      
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2],[5] $ 15,659,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period [5] 0                    
Office Buildings [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Initial Cost, Land [1]         342,605,000            
Initial Cost, Buildings and Improvements [1]         902,423,000            
Net Improvements (Retirement) since Acquisition         328,017,000            
Gross Amounts at Which Carried at December 31, 2011, Land         328,089,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         1,244,956,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] 1,573,045,000       1,573,045,000            
Accumulated Depreciation at December 31, 2011 437,262,000       437,262,000            
Net Rentable Square Feet | ft² [3]       4,869,000              
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] 1,573,045,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 437,262,000                    
1901 Pennsylvania Avenue [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Washington, DC                    
Initial Cost, Land [1]         892,000            
Initial Cost, Buildings and Improvements [1]         3,481,000            
Net Improvements (Retirement) since Acquisition         17,253,000            
Gross Amounts at Which Carried at December 31, 2011, Land         892,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         20,734,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 21,626,000       21,626,000            
Accumulated Depreciation at December 31, 2011 $ 15,589,000       15,589,000            
Year of Construction Jan. 01, 1960                    
Date of Acquisition May 01, 1977                    
Net Rentable Square Feet | ft²       101,000              
Depreciation Life [6] 28 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 21,626,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 15,589,000                    
51 Monroe Street [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Maryland                    
Initial Cost, Land [1]         840,000            
Initial Cost, Buildings and Improvements [1]         10,869,000            
Net Improvements (Retirement) since Acquisition         28,634,000            
Gross Amounts at Which Carried at December 31, 2011, Land         840,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         39,503,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 40,343,000       40,343,000            
Accumulated Depreciation at December 31, 2011 $ 29,387,000       29,387,000            
Year of Construction Jan. 01, 1975                    
Date of Acquisition Aug. 01, 1979                    
Net Rentable Square Feet | ft²       224,000              
Depreciation Life [6] 41 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 40,343,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 29,387,000                    
515 King Street [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Virginia                    
Initial Cost, Land [1]         4,102,000            
Initial Cost, Buildings and Improvements [1]         3,931,000            
Net Improvements (Retirement) since Acquisition         6,022,000            
Gross Amounts at Which Carried at December 31, 2011, Land         4,102,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         9,953,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 14,055,000       14,055,000            
Accumulated Depreciation at December 31, 2011 $ 5,416,000       5,416,000            
Year of Construction Jan. 01, 1966                    
Date of Acquisition Jul. 01, 1992                    
Net Rentable Square Feet | ft²       75,000              
Depreciation Life [6] 50 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 14,055,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 5,416,000                    
6110 Executive Boulevard [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Maryland                    
Initial Cost, Land [1]         4,621,000            
Initial Cost, Buildings and Improvements [1]         11,926,000            
Net Improvements (Retirement) since Acquisition         16,383,000            
Gross Amounts at Which Carried at December 31, 2011, Land         4,621,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         28,309,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 32,930,000       32,930,000            
Accumulated Depreciation at December 31, 2011 $ 19,283,000       19,283,000            
Year of Construction Jan. 01, 1971                    
Date of Acquisition Jan. 01, 1995                    
Net Rentable Square Feet | ft²       203,000              
Depreciation Life [6] 30 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 32,930,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 19,283,000                    
1220 19th Street [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Washington, DC                    
Initial Cost, Land [1]         7,803,000            
Initial Cost, Buildings and Improvements [1]         11,366,000            
Net Improvements (Retirement) since Acquisition         15,868,000            
Gross Amounts at Which Carried at December 31, 2011, Land         7,802,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         27,235,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 35,037,000       35,037,000            
Accumulated Depreciation at December 31, 2011 $ 13,982,000       13,982,000            
Year of Construction Jan. 01, 1976                    
Date of Acquisition Nov. 01, 1995                    
Net Rentable Square Feet | ft²       104,000              
Depreciation Life [6] 30 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 35,037,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 13,982,000                    
1600 Wilson Boulevard [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Virginia                    
Initial Cost, Land [1]         6,661,000            
Initial Cost, Buildings and Improvements [1]         16,742,000            
Net Improvements (Retirement) since Acquisition         26,685,000            
Gross Amounts at Which Carried at December 31, 2011, Land         6,661,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         43,427,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 50,088,000       50,088,000            
Accumulated Depreciation at December 31, 2011 $ 20,180,000       20,180,000            
Year of Construction Jan. 01, 1973                    
Date of Acquisition Oct. 01, 1997                    
Net Rentable Square Feet | ft²       168,000              
Depreciation Life [6] 30 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 50,088,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 20,180,000                    
7900 Westpark Drive [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location [8] Virginia                    
Initial Cost, Land [1],[8]         12,049,000            
Initial Cost, Buildings and Improvements [1]         71,825,000            
Net Improvements (Retirement) since Acquisition [8]         84,849,000            
Gross Amounts at Which Carried at December 31, 2011, Land [8]         12,049,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements [8]         156,674,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2],[8] $ 168,723,000       168,723,000            
Accumulated Depreciation at December 31, 2011 [8] $ 69,796,000       69,796,000            
Year of Construction Jan. 01, 1972                    
Date of Acquisition Nov. 01, 1997                    
Net Rentable Square Feet | ft²       531,000              
Depreciation Life [6] 30 years                    
Development in Process         10,200,000            
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2],[8] $ 168,723,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period [8] $ 69,796,000                    
600 Jefferson Plaza [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Maryland                    
Initial Cost, Land [1]         2,296,000            
Initial Cost, Buildings and Improvements [1]         12,188,000            
Net Improvements (Retirement) since Acquisition         7,040,000            
Gross Amounts at Which Carried at December 31, 2011, Land         2,296,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         19,228,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 21,524,000       21,524,000            
Accumulated Depreciation at December 31, 2011 $ 10,822,000       10,822,000            
Year of Construction Jan. 01, 1985                    
Date of Acquisition May 01, 1999                    
Net Rentable Square Feet | ft²       113,000              
Depreciation Life [6] 30 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 21,524,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 10,822,000                    
Wayne Plaza [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Maryland                    
Initial Cost, Land [1]         1,564,000            
Initial Cost, Buildings and Improvements [1]         6,243,000            
Net Improvements (Retirement) since Acquisition         9,295,000            
Gross Amounts at Which Carried at December 31, 2011, Land         1,564,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         15,538,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 17,102,000       17,102,000            
Accumulated Depreciation at December 31, 2011 $ 8,738,000       8,738,000            
Year of Construction Jan. 01, 1970                    
Date of Acquisition May 01, 2000                    
Net Rentable Square Feet | ft²       99,000              
Depreciation Life [6] 30 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 17,102,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 8,738,000                    
Courthouse Square [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Virginia                    
Initial Cost, Land [1]         0            
Initial Cost, Buildings and Improvements [1]         17,096,000            
Net Improvements (Retirement) since Acquisition         8,390,000            
Gross Amounts at Which Carried at December 31, 2011, Land         0            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         25,486,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 25,486,000       25,486,000            
Accumulated Depreciation at December 31, 2011 $ 13,084,000       13,084,000            
Year of Construction Jan. 01, 1979                    
Date of Acquisition Oct. 01, 2000                    
Net Rentable Square Feet | ft²       116,000              
Depreciation Life [6] 30 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 25,486,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 13,084,000                    
One Central Plaza [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Maryland                    
Initial Cost, Land [1]         5,480,000            
Initial Cost, Buildings and Improvements [1]         39,107,000            
Net Improvements (Retirement) since Acquisition         17,905,000            
Gross Amounts at Which Carried at December 31, 2011, Land         5,480,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         57,012,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 62,492,000       62,492,000            
Accumulated Depreciation at December 31, 2011 $ 30,982,000       30,982,000            
Year of Construction Jan. 01, 1974                    
Date of Acquisition Apr. 01, 2001                    
Net Rentable Square Feet | ft²       267,000              
Depreciation Life [6] 30 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 62,492,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 30,982,000                    
1776 G Street [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Washington, DC                    
Initial Cost, Land [1]         31,500,000            
Initial Cost, Buildings and Improvements [1]         54,327,000            
Net Improvements (Retirement) since Acquisition         5,774,000            
Gross Amounts at Which Carried at December 31, 2011, Land         31,500,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         60,101,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 91,601,000       91,601,000            
Accumulated Depreciation at December 31, 2011 $ 27,965,000       27,965,000            
Year of Construction Jan. 01, 1979                    
Date of Acquisition Aug. 01, 2003                    
Net Rentable Square Feet | ft²       265,000              
Depreciation Life [6] 30 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 91,601,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 27,965,000                    
Dulles Station II [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location [8] Virginia                    
Initial Cost, Land [1],[8]         15,001,000            
Initial Cost, Buildings and Improvements [1],[8]         494,000            
Net Improvements (Retirement) since Acquisition [8]         (3,400,000)            
Gross Amounts at Which Carried at December 31, 2011, Land [8]         484,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements [8]         11,611,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2],[8] $ 12,095,000       12,095,000            
Accumulated Depreciation at December 31, 2011 [8] $ 515,000       515,000            
Date of Acquisition [8] Dec. 01, 2005                    
Net Rentable Square Feet | ft²       0              
Area of land | ft²       360,000              
Land Placed In Service         3,600,000            
Development in Process         8,500,000            
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2],[8] $ 12,095,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period [8] $ 515,000                    
West Gude Drive [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Maryland                    
Initial Cost, Land [1]         11,580,000            
Initial Cost, Buildings and Improvements [1]         43,240,000            
Net Improvements (Retirement) since Acquisition         12,277,000            
Gross Amounts at Which Carried at December 31, 2011, Land         11,580,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         55,517,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 67,097,000       67,097,000            
Accumulated Depreciation at December 31, 2011 $ 20,184,000       20,184,000            
Year of Construction Jan. 01, 1984                    
Date of Acquisition Aug. 01, 2006                    
Net Rentable Square Feet | ft²       277,000              
Depreciation Life [6] 30 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 67,097,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 20,184,000                    
Monument II [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Virginia                    
Initial Cost, Land [1]         10,244,000            
Initial Cost, Buildings and Improvements [1]         65,205,000            
Net Improvements (Retirement) since Acquisition         8,089,000            
Gross Amounts at Which Carried at December 31, 2011, Land         10,244,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         73,294,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 83,538,000       83,538,000            
Accumulated Depreciation at December 31, 2011 $ 22,406,000       22,406,000            
Year of Construction Jan. 01, 2000                    
Date of Acquisition Mar. 01, 2007                    
Net Rentable Square Feet | ft²       208,000              
Depreciation Life [6] 30 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 83,538,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 22,406,000                    
2000 M Street [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Washington, DC                    
Initial Cost, Land [1]         0            
Initial Cost, Buildings and Improvements [1]         61,101,000            
Net Improvements (Retirement) since Acquisition         21,541,000            
Gross Amounts at Which Carried at December 31, 2011, Land         0            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         82,642,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 82,642,000       82,642,000            
Accumulated Depreciation at December 31, 2011 $ 24,244,000       24,244,000            
Year of Construction Jan. 01, 1971                    
Date of Acquisition Dec. 01, 2007                    
Net Rentable Square Feet | ft²       231,000              
Depreciation Life [6] 30 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 82,642,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 24,244,000                    
M Street 2445 [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location [5] Washington, DC                    
Initial Cost, Land [1],[5]         46,887,000            
Initial Cost, Buildings and Improvements [1],[5]         106,743,000            
Net Improvements (Retirement) since Acquisition [5]         6,163,000            
Gross Amounts at Which Carried at December 31, 2011, Land [5]         46,887,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements [5]         112,906,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2],[5] $ 159,793,000       159,793,000            
Accumulated Depreciation at December 31, 2011 [5] $ 31,690,000       31,690,000            
Year of Construction Jan. 01, 1986                    
Date of Acquisition Dec. 01, 2008                    
Net Rentable Square Feet | ft²       290,000              
Depreciation Life [6] 30 years                    
Mortgage notes payable         100,600,000            
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2],[5] $ 159,793,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period [5] $ 31,690,000                    
Quantico Building E [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Virginia                    
Initial Cost, Land [1]         4,518,000            
Initial Cost, Buildings and Improvements [1]         24,801,000            
Net Improvements (Retirement) since Acquisition         940,000            
Gross Amounts at Which Carried at December 31, 2011, Land         4,518,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         25,741,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 30,259,000       30,259,000            
Accumulated Depreciation at December 31, 2011 $ 7,475,000       7,475,000            
Year of Construction Jan. 01, 2007                    
Date of Acquisition Jun. 01, 2010                    
Net Rentable Square Feet | ft²       134,000              
Depreciation Life [6] 30 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 30,259,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 7,475,000                    
Quantico Building G [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Virginia                    
Initial Cost, Land [1]         4,897,000            
Initial Cost, Buildings and Improvements [1]         25,376,000            
Net Improvements (Retirement) since Acquisition         (357,000)            
Gross Amounts at Which Carried at December 31, 2011, Land         4,898,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         25,018,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 29,916,000       29,916,000            
Accumulated Depreciation at December 31, 2011 $ 6,824,000       6,824,000            
Year of Construction Jan. 01, 2009                    
Date of Acquisition Jun. 01, 2010                    
Net Rentable Square Feet | ft²       136,000              
Depreciation Life [6] 30 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 29,916,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 6,824,000                    
1140 Connecticut Avenue [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Washington, DC                    
Initial Cost, Land [1]         25,226,000            
Initial Cost, Buildings and Improvements [1]         50,495,000            
Net Improvements (Retirement) since Acquisition         12,102,000            
Gross Amounts at Which Carried at December 31, 2011, Land         25,226,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         62,597,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 87,823,000       87,823,000            
Accumulated Depreciation at December 31, 2011 $ 13,120,000       13,120,000            
Year of Construction Jan. 01, 1966                    
Date of Acquisition Jan. 01, 2011                    
Net Rentable Square Feet | ft²       183,000              
Depreciation Life [6] 30 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 87,823,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 13,120,000                    
1227 25th Street [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Washington, DC                    
Initial Cost, Land [1]         17,505,000            
Initial Cost, Buildings and Improvements [1]         21,319,000            
Net Improvements (Retirement) since Acquisition         2,875,000            
Gross Amounts at Which Carried at December 31, 2011, Land         17,505,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         24,194,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 41,699,000       41,699,000            
Accumulated Depreciation at December 31, 2011 $ 5,696,000       5,696,000            
Year of Construction Jan. 01, 1988                    
Date of Acquisition Mar. 01, 2011                    
Net Rentable Square Feet | ft²       135,000              
Depreciation Life [6] 30 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 41,699,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 5,696,000                    
Braddock Place Member                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Virginia                    
Initial Cost, Land [1]         18,817,000            
Initial Cost, Buildings and Improvements [1]         71,250,000            
Net Improvements (Retirement) since Acquisition         10,943,000            
Gross Amounts at Which Carried at December 31, 2011, Land         18,818,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         82,192,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 101,010,000       101,010,000            
Accumulated Depreciation at December 31, 2011 $ 16,263,000       16,263,000            
Year of Construction Jan. 01, 1985                    
Date of Acquisition Sep. 01, 2011                    
Net Rentable Square Feet | ft²       350,000              
Depreciation Life [6] 30 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 101,010,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 16,263,000                    
John Marshall II Member                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location [5] Virginia                    
Initial Cost, Land [1],[5]         13,490,000            
Initial Cost, Buildings and Improvements [1],[5]         53,024,000            
Net Improvements (Retirement) since Acquisition [5]         2,145,000            
Gross Amounts at Which Carried at December 31, 2011, Land [5]         13,490,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements [5]         55,169,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2],[5] $ 68,659,000       68,659,000            
Accumulated Depreciation at December 31, 2011 [5] $ 9,531,000       9,531,000            
Year of Construction Jan. 01, 1996                    
Date of Acquisition Sep. 01, 2011                    
Net Rentable Square Feet | ft²       223,000              
Depreciation Life [6] 30 years                    
Mortgage notes payable         50,100,000            
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2],[5] $ 68,659,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period [5] $ 9,531,000                    
Fairgate at Ballston [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Virginia                    
Initial Cost, Land [1]         17,750,000            
Initial Cost, Buildings and Improvements [1]         29,885,000            
Net Improvements (Retirement) since Acquisition         3,722,000            
Gross Amounts at Which Carried at December 31, 2011, Land         17,750,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         33,607,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 51,357,000       51,357,000            
Accumulated Depreciation at December 31, 2011 $ 5,835,000       5,835,000            
Year of Construction Jan. 01, 1988                    
Date of Acquisition Jun. 01, 2012                    
Net Rentable Square Feet | ft²       143,000              
Depreciation Life [6] 30 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 51,357,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 5,835,000                    
The Army Navy Club Building [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location [5] Washington, DC                    
Initial Cost, Land [1],[5]         30,796,000            
Initial Cost, Buildings and Improvements [1],[5]         39,315,000            
Net Improvements (Retirement) since Acquisition [5]         1,903,000            
Gross Amounts at Which Carried at December 31, 2011, Land [5]         30,796,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements [5]         41,218,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2],[5] $ 72,014,000       72,014,000            
Accumulated Depreciation at December 31, 2011 [5] $ 3,831,000       3,831,000            
Year of Construction Jan. 01, 1912                    
Date of Acquisition Mar. 01, 2014                    
Net Rentable Square Feet | ft²       108,000              
Depreciation Life [6] 30 years                    
Mortgage notes payable         51,000,000            
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2],[5] $ 72,014,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period [5] $ 3,831,000                    
1775 Eye Street [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Washington, DC                    
Initial Cost, Land [1]         48,086,000            
Initial Cost, Buildings and Improvements [1]         51,074,000            
Net Improvements (Retirement) since Acquisition         4,976,000            
Gross Amounts at Which Carried at December 31, 2011, Land         48,086,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         56,050,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 104,136,000       104,136,000            
Accumulated Depreciation at December 31, 2011 $ 4,424,000       4,424,000            
Year of Construction Jan. 01, 1964                    
Date of Acquisition May 01, 2014                    
Net Rentable Square Feet | ft²       185,000              
Depreciation Life [6] 30 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 104,136,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period 4,424,000                    
Retail Centers [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Initial Cost, Land [1]         120,969,000            
Initial Cost, Buildings and Improvements [1]         249,498,000            
Net Improvements (Retirement) since Acquisition         72,648,000            
Gross Amounts at Which Carried at December 31, 2011, Land         120,671,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         322,444,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] 443,115,000       443,115,000            
Accumulated Depreciation at December 31, 2011 114,146,000       114,146,000            
Net Rentable Square Feet | ft²       2,329,000              
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] 443,115,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 114,146,000                    
Takoma Park [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Maryland                    
Initial Cost, Land [1]         415,000            
Initial Cost, Buildings and Improvements [1]         1,084,000            
Net Improvements (Retirement) since Acquisition         281,000            
Gross Amounts at Which Carried at December 31, 2011, Land         415,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         1,365,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 1,780,000       1,780,000            
Accumulated Depreciation at December 31, 2011 $ 1,191,000       1,191,000            
Year of Construction Jan. 01, 1962                    
Date of Acquisition Jul. 01, 1963                    
Net Rentable Square Feet | ft²       51,000              
Depreciation Life [6] 50 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 1,780,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 1,191,000                    
Westminster [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Maryland                    
Initial Cost, Land [1]         519,000            
Initial Cost, Buildings and Improvements [1]         1,775,000            
Net Improvements (Retirement) since Acquisition         9,802,000            
Gross Amounts at Which Carried at December 31, 2011, Land         519,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         11,577,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 12,096,000       12,096,000            
Accumulated Depreciation at December 31, 2011 $ 7,268,000       7,268,000            
Year of Construction Jan. 01, 1969                    
Date of Acquisition Sep. 01, 1972                    
Net Rentable Square Feet | ft²       150,000              
Depreciation Life [6] 37 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 12,096,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 7,268,000                    
Concord Centre [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Virginia                    
Initial Cost, Land [1]         413,000            
Initial Cost, Buildings and Improvements [1]         850,000            
Net Improvements (Retirement) since Acquisition         5,456,000            
Gross Amounts at Which Carried at December 31, 2011, Land         413,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         6,306,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 6,719,000       6,719,000            
Accumulated Depreciation at December 31, 2011 $ 3,191,000       3,191,000            
Year of Construction Jan. 01, 1960                    
Date of Acquisition Dec. 01, 1973                    
Net Rentable Square Feet | ft²       76,000              
Depreciation Life [6] 33 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 6,719,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 3,191,000                    
Wheaton Park [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Maryland                    
Initial Cost, Land [1]         796,000            
Initial Cost, Buildings and Improvements [1]         857,000            
Net Improvements (Retirement) since Acquisition         4,665,000            
Gross Amounts at Which Carried at December 31, 2011, Land         796,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         5,522,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 6,318,000       6,318,000            
Accumulated Depreciation at December 31, 2011 $ 3,748,000       3,748,000            
Year of Construction Jan. 01, 1967                    
Date of Acquisition Sep. 01, 1977                    
Net Rentable Square Feet | ft²       74,000              
Depreciation Life [6] 50 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 6,318,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 3,748,000                    
Bradlee [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Virginia                    
Initial Cost, Land [1]         4,152,000            
Initial Cost, Buildings and Improvements [1]         5,383,000            
Net Improvements (Retirement) since Acquisition         10,941,000            
Gross Amounts at Which Carried at December 31, 2011, Land         4,152,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         16,324,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 20,476,000       20,476,000            
Accumulated Depreciation at December 31, 2011 $ 10,793,000       10,793,000            
Year of Construction Jan. 01, 1955                    
Date of Acquisition Dec. 01, 1984                    
Net Rentable Square Feet | ft²       171,000              
Depreciation Life [6] 40 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 20,476,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 10,793,000                    
Chevy Chase Metro Plaza [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Washington, DC                    
Initial Cost, Land [1]         1,549,000            
Initial Cost, Buildings and Improvements [1]         4,304,000            
Net Improvements (Retirement) since Acquisition         5,539,000            
Gross Amounts at Which Carried at December 31, 2011, Land         1,549,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         9,843,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 11,392,000       11,392,000            
Accumulated Depreciation at December 31, 2011 $ 6,621,000       6,621,000            
Year of Construction Jan. 01, 1975                    
Date of Acquisition Sep. 01, 1985                    
Net Rentable Square Feet | ft²       50,000              
Depreciation Life [6] 50 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 11,392,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 6,621,000                    
Shoppes of Foxchase [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Virginia                    
Initial Cost, Land [1]         5,838,000            
Initial Cost, Buildings and Improvements [1]         2,979,000            
Net Improvements (Retirement) since Acquisition         14,687,000            
Gross Amounts at Which Carried at December 31, 2011, Land         5,838,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         17,666,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 23,504,000       23,504,000            
Accumulated Depreciation at December 31, 2011 $ 6,631,000       6,631,000            
Year of Construction Jan. 01, 1960                    
Date of Acquisition Jun. 01, 1994                    
Net Rentable Square Feet | ft²       134,000              
Depreciation Life [6] 50 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 23,504,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 6,631,000                    
Frederick County Square [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Maryland                    
Initial Cost, Land [1]         6,561,000            
Initial Cost, Buildings and Improvements [1]         6,830,000            
Net Improvements (Retirement) since Acquisition         4,782,000            
Gross Amounts at Which Carried at December 31, 2011, Land         6,561,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         11,612,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 18,173,000       18,173,000            
Accumulated Depreciation at December 31, 2011 $ 7,349,000       7,349,000            
Year of Construction Jan. 01, 1973                    
Date of Acquisition Aug. 01, 1995                    
Net Rentable Square Feet | ft²       227,000              
Depreciation Life [6] 30 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 18,173,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 7,349,000                    
800 S. Washington Street [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Virginia                    
Initial Cost, Land [1]         2,904,000            
Initial Cost, Buildings and Improvements [1]         5,489,000            
Net Improvements (Retirement) since Acquisition         5,944,000            
Gross Amounts at Which Carried at December 31, 2011, Land         2,904,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         11,433,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 14,337,000       14,337,000            
Accumulated Depreciation at December 31, 2011 $ 4,761,000       4,761,000            
Year of Construction Jan. 01, 1951                    
Date of Acquisition Jun. 01, 1998                    
Net Rentable Square Feet | ft²       46,000              
Depreciation Life [6] 30 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 14,337,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 4,761,000                    
Centre at Hagerstown . [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Maryland                    
Initial Cost, Land [1]         13,029,000            
Initial Cost, Buildings and Improvements [1]         25,415,000            
Net Improvements (Retirement) since Acquisition         2,435,000            
Gross Amounts at Which Carried at December 31, 2011, Land         13,029,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         27,850,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 40,879,000       40,879,000            
Accumulated Depreciation at December 31, 2011 $ 12,855,000       12,855,000            
Year of Construction Jan. 01, 2000                    
Date of Acquisition Jun. 01, 2002                    
Net Rentable Square Feet | ft²       332,000              
Depreciation Life [6] 30 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 40,879,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 12,855,000                    
Frederick Crossing [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Maryland                    
Initial Cost, Land [1]         12,759,000            
Initial Cost, Buildings and Improvements [1]         35,477,000            
Net Improvements (Retirement) since Acquisition         2,242,000            
Gross Amounts at Which Carried at December 31, 2011, Land         12,759,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         37,719,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 50,478,000       50,478,000            
Accumulated Depreciation at December 31, 2011 $ 14,430,000       14,430,000            
Year of Construction Jan. 01, 1999                    
Date of Acquisition Mar. 01, 2005                    
Net Rentable Square Feet | ft²       295,000              
Depreciation Life [6] 30 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 50,478,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 14,430,000                    
Randolph Shopping Center [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Maryland                    
Initial Cost, Land [1]         4,928,000            
Initial Cost, Buildings and Improvements [1]         13,025,000            
Net Improvements (Retirement) since Acquisition         940,000            
Gross Amounts at Which Carried at December 31, 2011, Land         4,928,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         13,965,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 18,893,000       18,893,000            
Accumulated Depreciation at December 31, 2011 $ 4,785,000       4,785,000            
Year of Construction Jan. 01, 1972                    
Date of Acquisition May 01, 2006                    
Net Rentable Square Feet | ft²       84,000              
Depreciation Life [6] 30 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 18,893,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 4,785,000                    
Montrose Shopping Center [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Maryland                    
Initial Cost, Land [1]         11,612,000            
Initial Cost, Buildings and Improvements [1]         22,410,000            
Net Improvements (Retirement) since Acquisition         1,860,000            
Gross Amounts at Which Carried at December 31, 2011, Land         11,020,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         24,862,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 35,882,000       35,882,000            
Accumulated Depreciation at December 31, 2011 $ 8,490,000       8,490,000            
Year of Construction Jan. 01, 1970                    
Date of Acquisition May 01, 2006                    
Net Rentable Square Feet | ft²       145,000              
Depreciation Life [6] 30 years                    
Area of land | ft²             15,000        
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 35,882,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 8,490,000                    
Gateway Overlook [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Maryland                    
Initial Cost, Land [1]         28,816,000            
Initial Cost, Buildings and Improvements [1]         52,249,000            
Net Improvements (Retirement) since Acquisition         370,000            
Gross Amounts at Which Carried at December 31, 2011, Land         29,110,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         52,325,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 81,435,000       81,435,000            
Accumulated Depreciation at December 31, 2011 $ 13,734,000       13,734,000            
Year of Construction Jan. 01, 2007                    
Date of Acquisition Dec. 01, 2010                    
Net Rentable Square Feet | ft²       220,000              
Depreciation Life [6] 30 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 81,435,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 13,734,000                    
Olney Village Center Member                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location [5] Maryland                    
Initial Cost, Land [1],[5]         15,842,000            
Initial Cost, Buildings and Improvements [1],[5]         39,133,000            
Net Improvements (Retirement) since Acquisition [5]         1,836,000            
Gross Amounts at Which Carried at December 31, 2011, Land [5]         15,842,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements [5]         40,969,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2],[5] $ 56,811,000       56,811,000            
Accumulated Depreciation at December 31, 2011 [5] $ 6,731,000       6,731,000            
Year of Construction [5] Jan. 01, 1979                    
Date of Acquisition [5] Aug. 01, 2011                    
Net Rentable Square Feet | ft²       199,000              
Depreciation Life [6] 30 years                    
Mortgage notes payable         17,300,000            
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2],[5] $ 56,811,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period [5] $ 6,731,000                    
Spring Vally Retail Center [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Location Washington, DC                    
Initial Cost, Land [1]         10,836,000            
Initial Cost, Buildings and Improvements [1]         32,238,000            
Net Improvements (Retirement) since Acquisition         868,000            
Gross Amounts at Which Carried at December 31, 2011, Land         10,836,000            
Gross Amounts at Which Carried at December 31, 2011, Buildings and Improvements         33,106,000            
Gross Amounts at Which Carried at December 31, 2011, Total [2] $ 43,942,000       43,942,000            
Accumulated Depreciation at December 31, 2011 $ 1,568,000       $ 1,568,000            
Year of Construction Jan. 01, 1941                    
Date of Acquisition Oct. 01, 2014                    
Net Rentable Square Feet | ft²       75,000              
Depreciation Life [6] 30 years                    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]                      
Balance, end of period [2] $ 43,942,000                    
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]                      
Balance, end of period $ 1,568,000                    
Minimum [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Useful Life 3 years                    
Maximum [Member]                      
Real Estate and Accumulated Depreciation [Line Items]                      
Useful Life 50 years                    
[1] The purchase cost of real estate investments has been divided between land and buildings and improvements on the basis of management’s determination of the fair values.
[2] At December 31, 2015, total land, buildings and improvements are carried at $2,071.6 million for federal income tax purposes.
[3] Residential properties are presented in gross square feet.
[4] Includes non-cash accruals for capital items and assumed mortgages.
[5] At December 31, 2015, our properties were encumbered by non-recourse mortgage amounts as follows: $35.4 million on 3801 Connecticut Avenue, $16.5 million on Walker House, $29.1 million on Bethesda Hill, $33.6 million million on The Kenmore, $100.6 million on 2445 M Street, $50.1 million on John Marshall II, and $17.3 million on Olney Village Center, $51.9 million on Yale West, and $51.0 million on The Amy Navy Club Building. The Maxwell was encumbered by a construction loan with a 32.2 million million balance at December 31, 2015.
[6] The useful life shown is for the main structure. Buildings and improvements are depreciated over various useful lives ranging from 3 to 50 years.
[7] As of December 31, 2015, Washington REIT had under development a multifamily property in Arlington, Virginia. The value not yet placed into service at December 31, 2015 was $15.7 million.
[8] As of December 31, 2015, Washington REIT had under development an office project with 360,000 square feet of office space and a parking garage to be developed in Herndon, VA (Dulles Station, Phase II). The value not yet placed in service of Dulles Station, Phase II at December 31, 2015 was $8.5 million. $3.6 million of Dulles Station, Phase II was placed into service upon the completion of a portion of the parking garage structure. Additionally, Washington REIT had investments in various development or redevelopment projects, including Silverline Center. The value of this redevelopment not yet placed in service is $10.2 million at December 31, 2015.