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Real Estate and Accumulated Depreciation (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Real Estate Properties [Line Items]        
Initial Basis $ 935,811      
Cost capitalized subsequent to acquisition 867,389      
Land 450,256      
Buildings 1,352,944      
Total Purchase Price 1,803,200 $ 1,700,813 $ 1,622,710 $ 1,560,159
Accumulated Depreciation 488,166 458,279 $ 425,370 $ 396,617
Book Value 1,315,034 1,242,534    
Related Debt 897,888 783,400    
Development land, related debt 0 $ 68,672    
Total, related debt 904,528      
Shopping Centers        
Real Estate Properties [Line Items]        
Initial Basis 832,397      
Cost capitalized subsequent to acquisition 401,096      
Land 412,358      
Buildings 821,135      
Total Purchase Price 1,233,493      
Accumulated Depreciation 332,818      
Book Value 900,675      
Related Debt 639,385      
Shopping Centers | Ashburn Village, Ashburn, VA        
Real Estate Properties [Line Items]        
Initial Basis 11,431      
Cost capitalized subsequent to acquisition 20,278      
Land 6,764      
Buildings 24,945      
Total Purchase Price 31,709      
Accumulated Depreciation 12,993      
Book Value 18,716      
Related Debt $ 28,025      
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 7,503      
Land 1,178      
Buildings 7,503      
Total Purchase Price 8,681      
Accumulated Depreciation 1,764      
Book Value 6,917      
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 18,426      
Land 22,674      
Buildings 19,913      
Total Purchase Price 42,587      
Accumulated Depreciation 14,288      
Book Value 28,299      
Related Debt $ 39,968      
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      
Total Purchase Price 22,623      
Accumulated Depreciation 0      
Book Value 22,623      
Related Debt $ 10,877      
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 1,768      
Land 5,735      
Buildings 12,753      
Total Purchase Price 18,488      
Accumulated Depreciation 4,424      
Book Value 14,064      
Related Debt $ 9,948      
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 4,461      
Land 3,687      
Buildings 5,657      
Total Purchase Price 9,344      
Accumulated Depreciation 2,808      
Book Value 6,536      
Related Debt $ 16,730      
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 4,143      
Land 9,789      
Buildings 21,391      
Total Purchase Price 31,180      
Accumulated Depreciation 7,908      
Book Value 23,272      
Related Debt $ 13,543      
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 27,954      
Land 5,300      
Buildings 27,970      
Total Purchase Price 33,270      
Accumulated Depreciation 10,933      
Book Value 22,337      
Related Debt $ 16,082      
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 342      
Land 28,401      
Buildings 46,153      
Total Purchase Price 74,554      
Accumulated Depreciation 1,095      
Book Value 73,459      
Related Debt $ 39,140      
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 3,752      
Land 7,666      
Buildings 24,998      
Total Purchase Price 32,664      
Accumulated Depreciation 8,838      
Book Value 23,826      
Related Debt $ 13,529      
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      
Total Purchase Price 32,218      
Accumulated Depreciation 4,105      
Book Value 28,113      
Related Debt $ 17,086      
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 448      
Land 3,901      
Buildings 8,773      
Total Purchase Price 12,674      
Accumulated Depreciation 3,057      
Book Value 9,617      
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      
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 13,829      
Land 1,118      
Buildings 18,492      
Total Purchase Price 19,610      
Accumulated Depreciation 11,191      
Book Value 8,419      
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      
Total Purchase Price 2,601      
Accumulated Depreciation 297      
Book Value 2,304      
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 8,098      
Land 5,300      
Buildings 15,716      
Total Purchase Price 21,016      
Accumulated Depreciation 9,026      
Book Value 11,990      
Related Debt $ 7,696      
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 3,178      
Land 14,766      
Buildings 30,162      
Total Purchase Price 44,928      
Accumulated Depreciation 7,719      
Book Value 37,209      
Related Debt $ 12,577      
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 3,499      
Land 1,856      
Buildings 4,802      
Total Purchase Price 6,658      
Accumulated Depreciation 3,702      
Book Value 2,956      
Related Debt $ 15,859      
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 4,034      
Land 4,822      
Buildings 11,796      
Total Purchase Price 16,618      
Accumulated Depreciation 3,801      
Book Value 12,817      
Related Debt $ 5,649      
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 1,603      
Land 4,455      
Buildings 11,203      
Total Purchase Price 15,658      
Accumulated Depreciation 3,477      
Book Value 12,181      
Related Debt $ 7,325      
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 507      
Land 5,006      
Buildings 9,880      
Total Purchase Price 14,886      
Accumulated Depreciation 3,713      
Book Value 11,173      
Related Debt $ 6,423      
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 1,602      
Land 22,800      
Buildings 55,525      
Total Purchase Price 78,325      
Accumulated Depreciation 9,092      
Book Value 69,233      
Related Debt $ 36,507      
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 7,255      
Land 1,425      
Buildings 7,255      
Total Purchase Price 8,680      
Accumulated Depreciation 3,744      
Book Value 4,936      
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 37,312      
Land 6,546      
Buildings 37,311      
Total Purchase Price 43,857      
Accumulated Depreciation 13,727      
Book Value 30,130      
Related Debt $ 32,673      
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 6,243      
Land 1,132      
Buildings 7,529      
Total Purchase Price 8,661      
Accumulated Depreciation 5,934      
Book Value 2,727      
Related Debt $ 15,452      
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 11,220      
Land 950      
Buildings 14,670      
Total Purchase Price 15,620      
Accumulated Depreciation 12,751      
Book Value 2,869      
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 4,095      
Land 26,064      
Buildings 11,154      
Total Purchase Price 37,218      
Accumulated Depreciation 1,375      
Book Value 35,843      
Related Debt $ 14,135      
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 13,926      
Land 9,260      
Buildings 14,207      
Total Purchase Price 23,467      
Accumulated Depreciation 5,565      
Book Value 17,902      
Related Debt $ 12,029      
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 15,414      
Total Purchase Price 28,100      
Accumulated Depreciation 3,871      
Book Value 24,229      
Related Debt $ 14,950      
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 6,643      
Land 5,409      
Buildings 17,167      
Total Purchase Price 22,576      
Accumulated Depreciation 7,158      
Book Value 15,418      
Related Debt $ 9,783      
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 1,961      
Land 3,079      
Buildings 3,845      
Total Purchase Price 6,924      
Accumulated Depreciation 3,318      
Book Value 3,606      
Related Debt $ 11,613      
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 1,014      
Land 7,751      
Buildings 12,640      
Total Purchase Price 20,391      
Accumulated Depreciation 3,432      
Book Value 16,959      
Related Debt $ 9,999      
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 1,435      
Land 5,739      
Buildings 14,061      
Total Purchase Price 19,800      
Accumulated Depreciation 4,606      
Book Value 15,194      
Related Debt $ 8,244      
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 4,227      
Land 703      
Buildings 4,769      
Total Purchase Price 5,472      
Accumulated Depreciation 3,067      
Book Value 2,405      
Related Debt $ 14,537      
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      
Total Purchase Price 26,585      
Accumulated Depreciation 806      
Book Value 25,779      
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 1,553      
Land 43,863      
Buildings 8,839      
Total Purchase Price 52,702      
Accumulated Depreciation 5,656      
Book Value 47,046      
Related Debt 0      
Shopping Centers | Seabreeze Plaza, Palm Harbor, FL        
Real Estate Properties [Line Items]        
Initial Basis 24,526      
Cost capitalized subsequent to acquisition 1,960      
Land 8,665      
Buildings 17,821      
Total Purchase Price 26,486      
Accumulated Depreciation 5,611      
Book Value 20,875      
Related Debt $ 16,055      
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 203      
Land 1,147      
Buildings 1,976      
Total Purchase Price 3,123      
Accumulated Depreciation 491      
Book Value 2,632      
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 44,108      
Land 4,913      
Buildings 44,043      
Total Purchase Price 48,956      
Accumulated Depreciation 28,790      
Book Value 20,166      
Related Debt $ 64,472      
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 232      
Land 12,700      
Buildings 50,786      
Total Purchase Price 63,486      
Accumulated Depreciation 7,927      
Book Value 55,559      
Related Debt $ 32,016      
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 9,924      
Land 992      
Buildings 11,640      
Total Purchase Price 12,632      
Accumulated Depreciation 7,989      
Book Value 4,643      
Related Debt $ 11,154      
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 7,975      
Land 6,402      
Buildings 19,392      
Total Purchase Price 25,794      
Accumulated Depreciation 7,278      
Book Value 18,516      
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 24,345      
Land 15,254      
Buildings 27,986      
Total Purchase Price 43,240      
Accumulated Depreciation 21,333      
Book Value 21,907      
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 10,695      
Land 1,878      
Buildings 15,545      
Total Purchase Price 17,423      
Accumulated Depreciation 12,280      
Book Value 5,143      
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 4,359      
Land 703      
Buildings 6,130      
Total Purchase Price 6,833      
Accumulated Depreciation 4,792      
Book Value 2,041      
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 24,821      
Land 7,693      
Buildings 24,976      
Total Purchase Price 32,669      
Accumulated Depreciation 16,735      
Book Value 15,934      
Related Debt $ 37,998      
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 2,495      
Land 7,851      
Buildings 11,146      
Total Purchase Price 18,997      
Accumulated Depreciation 6,688      
Book Value 12,309      
Related Debt $ 13,438      
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 25,227      
Land 6,047      
Buildings 25,227      
Total Purchase Price 31,274      
Accumulated Depreciation 7,511      
Book Value 23,763      
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 5,366      
Land 4,649      
Buildings 6,994      
Total Purchase Price 11,643      
Accumulated Depreciation 6,043      
Book Value 5,600      
Related Debt $ 23,873      
Buildings and Improvements Depreciable Lives in Years 40 years      
Shopping Centers | Other Buildings / Improvements        
Real Estate Properties [Line Items]        
Cost capitalized subsequent to acquisition $ 423      
Buildings 423      
Total Purchase Price 423      
Accumulated Depreciation 109      
Book Value 314      
Related Debt 0      
Mixed-Use Properties        
Real Estate Properties [Line Items]        
Initial Basis 43,967      
Cost capitalized subsequent to acquisition 432,660      
Land 28,496      
Buildings 448,131      
Total Purchase Price 476,627      
Accumulated Depreciation 155,348      
Book Value 321,279      
Related Debt $ 265,143      
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 30,409      
Land 3,756      
Buildings 48,112      
Total Purchase Price 51,868      
Accumulated Depreciation 36,469      
Book Value 15,399      
Related Debt $ 28,115      
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 185,904      
Land 16,287      
Buildings 182,370      
Total Purchase Price 198,657      
Accumulated Depreciation 37,242      
Book Value 161,415      
Related Debt $ 105,817      
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 91,617      
Land 2,242      
Buildings 91,617      
Total Purchase Price 93,859      
Accumulated Depreciation 4,801      
Book Value 89,058      
Development land, related debt $ 71,211      
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 67,995      
Land 5,667      
Buildings 67,807      
Total Purchase Price 73,474      
Accumulated Depreciation 52,194      
Book Value 21,280      
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 56,735      
Land 544      
Buildings 58,225      
Total Purchase Price 58,769      
Accumulated Depreciation 24,642      
Book Value 34,127      
Related Debt $ 60,000      
Buildings and Improvements Depreciable Lives in Years 50 years      
Development Land        
Real Estate Properties [Line Items]        
Initial Basis $ 59,447      
Cost capitalized subsequent to acquisition 33,633      
Land 9,402      
Buildings 83,678      
Total Purchase Price 93,080      
Accumulated Depreciation 0      
Book Value 93,080      
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 1,917      
Land 7,028      
Buildings 181      
Total Purchase Price 7,209      
Accumulated Depreciation 0      
Book Value 7,209      
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      
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 31,430      
Land 0      
Buildings 83,497      
Total Purchase Price 83,497      
Accumulated Depreciation 0      
Book Value 83,497      
Development land, related debt $ 0