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Real Estate and Accumulated Depreciation (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Real Estate Properties [Line Items]        
Initial Basis $ 984,390      
Cost capitalized subsequent to acquisition 963,775      
Land 488,918      
Buildings 1,273,275      
Construction in progress 185,972 $ 91,114    
Total Purchase Price 1,948,165 1,803,200 $ 1,700,813 $ 1,622,710
Accumulated Depreciation 525,518 488,166 $ 458,279 $ 425,370
Book Value 1,422,647 1,315,034    
Related Debt 880,271 897,888    
Development land, related debt 21,655 $ 0    
Total, related debt 910,189      
Shopping Centers        
Real Estate Properties [Line Items]        
Initial Basis 832,397      
Cost capitalized subsequent to acquisition 413,662      
Land 412,358      
Buildings 822,249      
Construction in progress 11,452      
Total Purchase Price 1,246,059      
Accumulated Depreciation 357,235      
Book Value 888,824      
Related Debt 629,111      
Shopping Centers | Ashburn Village, Ashburn, VA        
Real Estate Properties [Line Items]        
Initial Basis 11,431      
Cost capitalized subsequent to acquisition 20,391      
Land 6,764      
Buildings 24,957      
Construction in progress 101      
Total Purchase Price 31,822      
Accumulated Depreciation 13,437      
Book Value 18,385      
Related Debt $ 27,167      
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | Ashland Square Phase I, Dumfries, VA        
Real Estate Properties [Line Items]        
Initial Basis $ 1,178      
Cost capitalized subsequent to acquisition 7,508      
Land 1,178      
Buildings 5,297      
Construction in progress 2,211      
Total Purchase Price 8,686      
Accumulated Depreciation 1,991      
Book Value 6,695      
Related Debt $ 0      
Shopping Centers | Ashland Square Phase I, Dumfries, VA | Minimum        
Real Estate Properties [Line Items]        
Buildings and Improvements Depreciable Lives in Years 20 years      
Shopping Centers | Ashland Square Phase I, Dumfries, VA | Maximum        
Real Estate Properties [Line Items]        
Buildings and Improvements Depreciable Lives in Years 50 years      
Shopping Centers | Beacon Center, Alexandria, VA        
Real Estate Properties [Line Items]        
Initial Basis $ 24,161      
Cost capitalized subsequent to acquisition 18,466      
Land 22,674      
Buildings 19,934      
Construction in progress 19      
Total Purchase Price 42,627      
Accumulated Depreciation 14,814      
Book Value 27,813      
Related Debt $ 38,120      
Shopping Centers | Beacon Center, Alexandria, VA | Minimum        
Real Estate Properties [Line Items]        
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | Beacon Center, Alexandria, VA | Maximum        
Real Estate Properties [Line Items]        
Buildings and Improvements Depreciable Lives in Years 50 years      
Shopping Centers | BJ’s Wholesale Club, Alexandria, VA        
Real Estate Properties [Line Items]        
Initial Basis $ 22,623      
Cost capitalized subsequent to acquisition 0      
Land 22,623      
Buildings 0      
Construction in progress 0      
Total Purchase Price 22,623      
Accumulated Depreciation 0      
Book Value 22,623      
Related Debt $ 10,609      
Buildings and Improvements Depreciable Lives in Years 0 years      
Shopping Centers | Boca Valley Plaza, Boca Raton, FL        
Real Estate Properties [Line Items]        
Initial Basis $ 16,720      
Cost capitalized subsequent to acquisition 1,756      
Land 5,735      
Buildings 12,741      
Construction in progress 0      
Total Purchase Price 18,476      
Accumulated Depreciation 4,837      
Book Value 13,639      
Related Debt $ 9,601      
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | Boulevard, Fairfax, VA        
Real Estate Properties [Line Items]        
Initial Basis $ 4,883      
Cost capitalized subsequent to acquisition 4,676      
Land 3,687      
Buildings 5,872      
Construction in progress 0      
Total Purchase Price 9,559      
Accumulated Depreciation 3,023      
Book Value 6,536      
Related Debt $ 16,236      
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | Briggs Chaney MarketPlace, Silver Spring, MD        
Real Estate Properties [Line Items]        
Initial Basis $ 27,037      
Cost capitalized subsequent to acquisition 4,169      
Land 9,789      
Buildings 21,417      
Construction in progress 0      
Total Purchase Price 31,206      
Accumulated Depreciation 8,606      
Book Value 22,600      
Related Debt $ 12,714      
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | Broadlands Village, Ashburn, VA        
Real Estate Properties [Line Items]        
Initial Basis $ 5,316      
Cost capitalized subsequent to acquisition 29,223      
Land 5,300      
Buildings 29,226      
Construction in progress 13      
Total Purchase Price 34,539      
Accumulated Depreciation 11,929      
Book Value 22,610      
Related Debt $ 31,941      
Shopping Centers | Broadlands Village, Ashburn, VA | Minimum        
Real Estate Properties [Line Items]        
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | Broadlands Village, Ashburn, VA | Maximum        
Real Estate Properties [Line Items]        
Buildings and Improvements Depreciable Lives in Years 50 years      
Shopping Centers | Burtonsville Town Square, Burtonsville, MD        
Real Estate Properties [Line Items]        
Initial Basis $ 74,212      
Cost capitalized subsequent to acquisition 4,800      
Land 28,401      
Buildings 49,472      
Construction in progress 1,139      
Total Purchase Price 79,012      
Accumulated Depreciation 2,324      
Book Value 76,688      
Related Debt $ 38,076      
Shopping Centers | Burtonsville Town Square, Burtonsville, MD | Minimum        
Real Estate Properties [Line Items]        
Buildings and Improvements Depreciable Lives in Years 20 years      
Shopping Centers | Burtonsville Town Square, Burtonsville, MD | Maximum        
Real Estate Properties [Line Items]        
Buildings and Improvements Depreciable Lives in Years 45 years      
Shopping Centers | Countryside Marketplace, Sterling, VA        
Real Estate Properties [Line Items]        
Initial Basis $ 28,912      
Cost capitalized subsequent to acquisition 3,811      
Land 7,666      
Buildings 25,045      
Construction in progress 12      
Total Purchase Price 32,723      
Accumulated Depreciation 9,666      
Book Value 23,057      
Related Debt $ 12,676      
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | Cranberry Square, Westminster, MD        
Real Estate Properties [Line Items]        
Initial Basis $ 31,578      
Cost capitalized subsequent to acquisition 640      
Land 6,700      
Buildings 25,518      
Construction in progress 0      
Total Purchase Price 32,218      
Accumulated Depreciation 4,802      
Book Value 27,416      
Related Debt $ 16,515      
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | Cruse MarketPlace, Cumming, GA        
Real Estate Properties [Line Items]        
Initial Basis $ 12,226      
Cost capitalized subsequent to acquisition 521      
Land 3,901      
Buildings 8,846      
Construction in progress 0      
Total Purchase Price 12,747      
Accumulated Depreciation 3,335      
Book Value 9,412      
Related Debt $ 0      
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | Flagship Center, Rockville, MD        
Real Estate Properties [Line Items]        
Initial Basis $ 160      
Cost capitalized subsequent to acquisition 9      
Land 169      
Buildings 0      
Construction in progress 0      
Total Purchase Price 169      
Accumulated Depreciation 0      
Book Value 169      
Related Debt $ 0      
Buildings and Improvements Depreciable Lives in Years 0 years      
Shopping Centers | French Market, Oklahoma City, OK        
Real Estate Properties [Line Items]        
Initial Basis $ 5,781      
Cost capitalized subsequent to acquisition 14,174      
Land 1,118      
Buildings 18,834      
Construction in progress 3      
Total Purchase Price 19,955      
Accumulated Depreciation 11,846      
Book Value 8,109      
Related Debt $ 0      
Buildings and Improvements Depreciable Lives in Years 50 years      
Shopping Centers | Germantown, Germantown, MD        
Real Estate Properties [Line Items]        
Initial Basis $ 2,034      
Cost capitalized subsequent to acquisition 567      
Land 2,034      
Buildings 567      
Construction in progress 0      
Total Purchase Price 2,601      
Accumulated Depreciation 353      
Book Value 2,248      
Related Debt $ 0      
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | The Glen, Woodbridge, VA        
Real Estate Properties [Line Items]        
Initial Basis $ 12,918      
Cost capitalized subsequent to acquisition 8,192      
Land 5,300      
Buildings 15,810      
Construction in progress 0      
Total Purchase Price 21,110      
Accumulated Depreciation 9,582      
Book Value 11,528      
Related Debt $ 22,900      
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | Great Falls Center, Great Falls, VA        
Real Estate Properties [Line Items]        
Initial Basis $ 41,750      
Cost capitalized subsequent to acquisition 3,192      
Land 14,766      
Buildings 30,174      
Construction in progress 2      
Total Purchase Price 44,942      
Accumulated Depreciation 8,612      
Book Value 36,330      
Related Debt $ 11,702      
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | Hampshire Langley, Takoma, MD        
Real Estate Properties [Line Items]        
Initial Basis $ 3,159      
Cost capitalized subsequent to acquisition 3,527      
Land 1,856      
Buildings 4,635      
Construction in progress 195      
Total Purchase Price 6,686      
Accumulated Depreciation 3,805      
Book Value 2,881      
Related Debt $ 15,345      
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | Hunt Club Corners, Apopka, FL        
Real Estate Properties [Line Items]        
Initial Basis $ 12,584      
Cost capitalized subsequent to acquisition 4,226      
Land 4,822      
Buildings 11,988      
Construction in progress 0      
Total Purchase Price 16,810      
Accumulated Depreciation 4,299      
Book Value 12,511      
Related Debt $ 5,480      
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | Jamestown Place, Altamonte Springs, FL        
Real Estate Properties [Line Items]        
Initial Basis $ 14,055      
Cost capitalized subsequent to acquisition 1,861      
Land 4,455      
Buildings 11,461      
Construction in progress 0      
Total Purchase Price 15,916      
Accumulated Depreciation 3,848      
Book Value 12,068      
Related Debt $ 6,943      
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | Kentlands Square I, Gaithersburg, MD        
Real Estate Properties [Line Items]        
Initial Basis $ 14,379      
Cost capitalized subsequent to acquisition 665      
Land 5,006      
Buildings 9,701      
Construction in progress 337      
Total Purchase Price 15,044      
Accumulated Depreciation 3,966      
Book Value 11,078      
Related Debt $ 0      
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | Kentlands Square II, Gaithersburg, MD        
Real Estate Properties [Line Items]        
Initial Basis $ 76,723      
Cost capitalized subsequent to acquisition 1,628      
Land 22,800      
Buildings 55,184      
Construction in progress 367      
Total Purchase Price 78,351      
Accumulated Depreciation 10,651      
Book Value 67,700      
Related Debt $ 35,258      
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | Kentlands Place, Gaithersburg, MD        
Real Estate Properties [Line Items]        
Initial Basis $ 1,425      
Cost capitalized subsequent to acquisition 7,298      
Land 1,425      
Buildings 7,298      
Construction in progress 0      
Total Purchase Price 8,723      
Accumulated Depreciation 3,988      
Book Value 4,735      
Related Debt $ 0      
Buildings and Improvements Depreciable Lives in Years 50 years      
Shopping Centers | Lansdowne Town Center, Leesburg, VA        
Real Estate Properties [Line Items]        
Initial Basis $ 6,545      
Cost capitalized subsequent to acquisition 38,528      
Land 6,546      
Buildings 37,702      
Construction in progress 825      
Total Purchase Price 45,073      
Accumulated Depreciation 15,011      
Book Value 30,062      
Related Debt $ 31,723      
Buildings and Improvements Depreciable Lives in Years 50 years      
Shopping Centers | Leesburg Pike Plaza, Baileys Crossroads, VA        
Real Estate Properties [Line Items]        
Initial Basis $ 2,418      
Cost capitalized subsequent to acquisition 6,262      
Land 1,132      
Buildings 7,548      
Construction in progress 0      
Total Purchase Price 8,680      
Accumulated Depreciation 6,054      
Book Value 2,626      
Related Debt $ 14,952      
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | Lumberton Plaza, Lumberton, NJ        
Real Estate Properties [Line Items]        
Initial Basis $ 4,400      
Cost capitalized subsequent to acquisition 11,305      
Land 950      
Buildings 14,754      
Construction in progress 1      
Total Purchase Price 15,705      
Accumulated Depreciation 13,062      
Book Value 2,643      
Related Debt $ 0      
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | Metro Pike Center, Rockville, MD        
Real Estate Properties [Line Items]        
Initial Basis $ 33,123      
Cost capitalized subsequent to acquisition 4,235      
Land 26,064      
Buildings 7,411      
Construction in progress 3,883      
Total Purchase Price 37,358      
Accumulated Depreciation 1,587      
Book Value 35,771      
Related Debt $ 0      
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | Shops at Monocacy, Frederick, MD        
Real Estate Properties [Line Items]        
Initial Basis $ 9,541      
Cost capitalized subsequent to acquisition 13,957      
Land 9,260      
Buildings 14,238      
Construction in progress 0      
Total Purchase Price 23,498      
Accumulated Depreciation 5,999      
Book Value 17,499      
Related Debt $ 11,295      
Buildings and Improvements Depreciable Lives in Years 50 years      
Shopping Centers | Northrock, Warrenton, VA        
Real Estate Properties [Line Items]        
Initial Basis $ 12,686      
Cost capitalized subsequent to acquisition 15,414      
Land 12,686      
Buildings 15,408      
Construction in progress 6      
Total Purchase Price 28,100      
Accumulated Depreciation 4,406      
Book Value 23,694      
Related Debt $ 14,526      
Buildings and Improvements Depreciable Lives in Years 50 years      
Shopping Centers | Olde Forte Village, Ft. Washington, MD        
Real Estate Properties [Line Items]        
Initial Basis $ 15,933      
Cost capitalized subsequent to acquisition 6,669      
Land 5,409      
Buildings 17,193      
Construction in progress 0      
Total Purchase Price 22,602      
Accumulated Depreciation 7,672      
Book Value 14,930      
Related Debt $ 9,159      
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | Olney, Olney, MD        
Real Estate Properties [Line Items]        
Initial Basis $ 4,963      
Cost capitalized subsequent to acquisition 2,106      
Land 3,079      
Buildings 3,990      
Construction in progress 0      
Total Purchase Price 7,069      
Accumulated Depreciation 3,390      
Book Value 3,679      
Related Debt $ 11,781      
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | Orchard Park, Dunwoody, GA        
Real Estate Properties [Line Items]        
Initial Basis $ 19,377      
Cost capitalized subsequent to acquisition 1,029      
Land 7,751      
Buildings 12,655      
Construction in progress 0      
Total Purchase Price 20,406      
Accumulated Depreciation 3,802      
Book Value 16,604      
Related Debt $ 9,728      
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | Palm Springs Center, Altamonte Springs, FL        
Real Estate Properties [Line Items]        
Initial Basis $ 18,365      
Cost capitalized subsequent to acquisition 1,938      
Land 5,739      
Buildings 14,564      
Construction in progress 0      
Total Purchase Price 20,303      
Accumulated Depreciation 5,088      
Book Value 15,215      
Related Debt $ 7,766      
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | Ravenwood, Baltimore, MD        
Real Estate Properties [Line Items]        
Initial Basis $ 1,245      
Cost capitalized subsequent to acquisition 4,193      
Land 703      
Buildings 4,735      
Construction in progress 0      
Total Purchase Price 5,438      
Accumulated Depreciation 3,171      
Book Value 2,267      
Related Debt $ 14,086      
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | 11503 Rockville Pike/5541 Nicholson Lane, Rockville, MD        
Real Estate Properties [Line Items]        
Initial Basis $ 26,561      
Cost capitalized subsequent to acquisition 24      
Land 22,113      
Buildings 4,472      
Construction in progress 0      
Total Purchase Price 26,585      
Accumulated Depreciation 918      
Book Value 25,667      
Related Debt $ 0      
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | 1500/1580/1582/1584 Rockville Pike, Rockville, MD        
Real Estate Properties [Line Items]        
Initial Basis $ 51,149      
Cost capitalized subsequent to acquisition 2,238      
Land 43,863      
Buildings 7,483      
Construction in progress 2,041      
Total Purchase Price 53,387      
Accumulated Depreciation 6,121      
Book Value 47,266      
Related Debt 0      
Shopping Centers | Seabreeze Plaza, Palm Harbor, FL        
Real Estate Properties [Line Items]        
Initial Basis 24,526      
Cost capitalized subsequent to acquisition 2,315      
Land 8,665      
Buildings 18,176      
Construction in progress 0      
Total Purchase Price 26,841      
Accumulated Depreciation 6,169      
Book Value 20,672      
Related Debt $ 15,547      
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | Market Place at Sea Colony, Bethany Beach, DE        
Real Estate Properties [Line Items]        
Initial Basis $ 2,920      
Cost capitalized subsequent to acquisition 209      
Land 1,147      
Buildings 1,982      
Construction in progress 0      
Total Purchase Price 3,129      
Accumulated Depreciation 556      
Book Value 2,573      
Related Debt $ 0      
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | Seven Corners, Falls Church, VA        
Real Estate Properties [Line Items]        
Initial Basis $ 4,848      
Cost capitalized subsequent to acquisition 44,101      
Land 4,913      
Buildings 43,971      
Construction in progress 65      
Total Purchase Price 48,949      
Accumulated Depreciation 29,867      
Book Value 19,082      
Related Debt $ 62,630      
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | Severna Park Marketplace, Severna Park, MD        
Real Estate Properties [Line Items]        
Initial Basis $ 63,254      
Cost capitalized subsequent to acquisition 512      
Land 12,700      
Buildings 51,066      
Construction in progress 0      
Total Purchase Price 63,766      
Accumulated Depreciation 9,233      
Book Value 54,533      
Related Debt $ 30,888      
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | Shops at Fairfax, Fairfax, VA        
Real Estate Properties [Line Items]        
Initial Basis $ 2,708      
Cost capitalized subsequent to acquisition 9,969      
Land 992      
Buildings 11,685      
Construction in progress 0      
Total Purchase Price 12,677      
Accumulated Depreciation 8,343      
Book Value 4,334      
Related Debt $ 10,824      
Buildings and Improvements Depreciable Lives in Years 50 years      
Shopping Centers | Smallwood Village Center, Waldorf, MD        
Real Estate Properties [Line Items]        
Initial Basis $ 17,819      
Cost capitalized subsequent to acquisition 8,218      
Land 6,402      
Buildings 19,629      
Construction in progress 6      
Total Purchase Price 26,037      
Accumulated Depreciation 8,087      
Book Value 17,950      
Related Debt $ 0      
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | Southdale, Glen Burnie, MD        
Real Estate Properties [Line Items]        
Initial Basis $ 18,895      
Cost capitalized subsequent to acquisition 24,825      
Land 15,254      
Buildings 28,465      
Construction in progress 1      
Total Purchase Price 43,720      
Accumulated Depreciation 21,947      
Book Value 21,773      
Related Debt $ 0      
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | Southside Plaza, Richmond, VA        
Real Estate Properties [Line Items]        
Initial Basis $ 6,728      
Cost capitalized subsequent to acquisition 10,998      
Land 1,878      
Buildings 15,848      
Construction in progress 0      
Total Purchase Price 17,726      
Accumulated Depreciation 12,466      
Book Value 5,260      
Related Debt $ 0      
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | South Dekalb Plaza, Atlanta, GA        
Real Estate Properties [Line Items]        
Initial Basis $ 2,474      
Cost capitalized subsequent to acquisition 4,417      
Land 703      
Buildings 6,188      
Construction in progress 0      
Total Purchase Price 6,891      
Accumulated Depreciation 4,942      
Book Value 1,949      
Related Debt $ 0      
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | Thruway, Winston-Salem, NC        
Real Estate Properties [Line Items]        
Initial Basis $ 7,848      
Cost capitalized subsequent to acquisition 25,568      
Land 7,693      
Buildings 25,679      
Construction in progress 44      
Total Purchase Price 33,416      
Accumulated Depreciation 17,777      
Book Value 15,639      
Related Debt $ 36,711      
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | Village Center, Centreville, VA        
Real Estate Properties [Line Items]        
Initial Basis $ 16,502      
Cost capitalized subsequent to acquisition 2,501      
Land 7,851      
Buildings 11,152      
Construction in progress 0      
Total Purchase Price 19,003      
Accumulated Depreciation 6,995      
Book Value 12,008      
Related Debt $ 13,014      
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | Westview Village, Frederick, MD        
Real Estate Properties [Line Items]        
Initial Basis $ 6,047      
Cost capitalized subsequent to acquisition 25,265      
Land 6,047      
Buildings 25,262      
Construction in progress 3      
Total Purchase Price 31,312      
Accumulated Depreciation 8,590      
Book Value 22,722      
Related Debt $ 0      
Buildings and Improvements Depreciable Lives in Years 50 years      
Shopping Centers | White Oak, Silver Spring, MD        
Real Estate Properties [Line Items]        
Initial Basis $ 6,277      
Cost capitalized subsequent to acquisition 5,387      
Land 4,649      
Buildings 6,837      
Construction in progress 178      
Total Purchase Price 11,664      
Accumulated Depreciation 6,169      
Book Value 5,495      
Related Debt $ 23,198      
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | Other Buildings / Improvements        
Real Estate Properties [Line Items]        
Cost capitalized subsequent to acquisition $ 179      
Buildings 179      
Construction in progress 0      
Total Purchase Price 179      
Accumulated Depreciation 99      
Book Value 80      
Related Debt 0      
Mixed-Use Properties        
Real Estate Properties [Line Items]        
Initial Basis 83,608      
Cost capitalized subsequent to acquisition 435,509      
Land 67,158      
Buildings 451,026      
Construction in progress 933      
Total Purchase Price 519,117      
Accumulated Depreciation 168,283      
Book Value 350,834      
Related Debt $ 257,746      
Mixed-Use Properties | 1500/1580/1582/1584 Rockville Pike, Rockville, MD | Acquisition Date One        
Real Estate Properties [Line Items]        
Buildings and Improvements Depreciable Lives in Years 5 years      
Mixed-Use Properties | 1500/1580/1582/1584 Rockville Pike, Rockville, MD | Acquisition Date Two        
Real Estate Properties [Line Items]        
Buildings and Improvements Depreciable Lives in Years 10 years      
Mixed-Use Properties | 1500/1580/1582/1584 Rockville Pike, Rockville, MD | Acquisition Date Three        
Real Estate Properties [Line Items]        
Buildings and Improvements Depreciable Lives in Years 5 years      
Mixed-Use Properties | 1500/1580/1582/1584 Rockville Pike, Rockville, MD | Acquisition Date Four        
Real Estate Properties [Line Items]        
Buildings and Improvements Depreciable Lives in Years 4 years      
Mixed-Use Properties | Avenel Business Park, Gaithersburg, MD        
Real Estate Properties [Line Items]        
Initial Basis $ 21,459      
Cost capitalized subsequent to acquisition 31,401      
Land 3,756      
Buildings 49,104      
Construction in progress 0      
Total Purchase Price 52,860      
Accumulated Depreciation 37,712      
Book Value 15,148      
Related Debt $ 27,222      
Mixed-Use Properties | Avenel Business Park, Gaithersburg, MD | Minimum        
Real Estate Properties [Line Items]        
Buildings and Improvements Depreciable Lives in Years 35 years      
Mixed-Use Properties | Avenel Business Park, Gaithersburg, MD | Maximum        
Real Estate Properties [Line Items]        
Buildings and Improvements Depreciable Lives in Years 40 years      
Mixed-Use Properties | Clarendon Center, Arlington, VA (1)        
Real Estate Properties [Line Items]        
Initial Basis $ 12,753      
Cost capitalized subsequent to acquisition 186,038      
Land 16,287      
Buildings 182,504      
Construction in progress 0      
Total Purchase Price 198,791      
Accumulated Depreciation 41,919      
Book Value 156,872      
Related Debt $ 102,310      
Buildings and Improvements Depreciable Lives in Years 50 years      
Mixed-Use Properties | Park Van Ness, Washington, DC        
Real Estate Properties [Line Items]        
Initial Basis $ 2,242      
Cost capitalized subsequent to acquisition 91,690      
Land 2,242      
Buildings 91,690      
Construction in progress 0      
Total Purchase Price 93,932      
Accumulated Depreciation 7,778      
Book Value 86,154      
Development land, related debt $ 69,691      
Buildings and Improvements Depreciable Lives in Years 50 years      
Mixed-Use Properties | 601 Pennsylvania Ave., Washington, DC        
Real Estate Properties [Line Items]        
Initial Basis $ 5,479      
Cost capitalized subsequent to acquisition 68,712      
Land 5,667      
Buildings 68,524      
Construction in progress 0      
Total Purchase Price 74,191      
Accumulated Depreciation 54,652      
Book Value 19,539      
Related Debt $ 0      
Buildings and Improvements Depreciable Lives in Years 35 years      
Mixed-Use Properties | Washington Square, Alexandria, VA        
Real Estate Properties [Line Items]        
Initial Basis $ 2,034      
Cost capitalized subsequent to acquisition 56,734      
Land 544      
Buildings 58,224      
Construction in progress 0      
Total Purchase Price 58,768      
Accumulated Depreciation 26,127      
Book Value 32,641      
Related Debt $ 58,523      
Buildings and Improvements Depreciable Lives in Years 50 years      
Mixed-Use Properties | 7316 Wisconsin Avenue        
Real Estate Properties [Line Items]        
Initial Basis $ 39,641      
Cost capitalized subsequent to acquisition 934      
Land 38,662      
Buildings 980      
Construction in progress 933      
Total Purchase Price 40,575      
Accumulated Depreciation 95      
Book Value 40,480      
Related Debt $ 0      
Buildings and Improvements Depreciable Lives in Years 2 years      
Development Land        
Real Estate Properties [Line Items]        
Initial Basis $ 68,385      
Cost capitalized subsequent to acquisition 114,604      
Land 9,402      
Buildings 0      
Construction in progress 173,587      
Total Purchase Price 182,989      
Accumulated Depreciation 0      
Book Value 182,989      
Development land, related debt 23,332      
Development Land | Ashbrook Marketplace, Ashburn, VA        
Real Estate Properties [Line Items]        
Initial Basis 8,938      
Cost capitalized subsequent to acquisition 2,186      
Land 0      
Buildings 0      
Construction in progress 11,124      
Total Purchase Price 11,124      
Accumulated Depreciation 0      
Book Value 11,124      
Development land, related debt 0      
Development Land | Ashland Square Phase II, Manassas, VA        
Real Estate Properties [Line Items]        
Initial Basis 5,292      
Cost capitalized subsequent to acquisition 2,023      
Land 7,028      
Buildings 0      
Construction in progress 287      
Total Purchase Price 7,315      
Accumulated Depreciation 0      
Book Value 7,315      
Development land, related debt 0      
Development Land | New Market, New Market, MD        
Real Estate Properties [Line Items]        
Initial Basis 2,088      
Cost capitalized subsequent to acquisition 286      
Land 2,374      
Buildings 0      
Construction in progress 0      
Total Purchase Price 2,374      
Accumulated Depreciation 0      
Book Value 2,374      
Development land, related debt 0      
Development Land | North Glebe Road, Arlington, VA        
Real Estate Properties [Line Items]        
Initial Basis 52,067      
Cost capitalized subsequent to acquisition 110,109      
Land 0      
Buildings 0      
Construction in progress 162,176      
Total Purchase Price 162,176      
Accumulated Depreciation 0      
Book Value 162,176      
Development land, related debt $ 23,332