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Schedule III - Real Estate and Accumulated Depreciation (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 320,107      
Initial Cost [Abstract]        
Land 449,911      
Buildings and Improvements 2,815,831      
Net Improvements (Retirements) Since Acquisition 566,606      
Carrying Amount at Which Carried [Abstract]        
Land 492,197      
Buildings and Improvements 3,340,151      
Total 3,832,348 $ 3,659,886 $ 4,487,588 $ 4,631,128
Accumulated Depreciation at December 31, 2017 895,091 $ 885,392 $ 1,080,616 $ 1,078,996
PENNSYLVANIA SUBURBS [Member] | 400 Berwyn Park [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances 0      
Initial Cost [Abstract]        
Land 2,657      
Buildings and Improvements 4,462      
Net Improvements (Retirements) Since Acquisition 12,982      
Carrying Amount at Which Carried [Abstract]        
Land 2,657      
Buildings and Improvements 17,444      
Total 20,101      
Accumulated Depreciation at December 31, 2017 $ 7,366      
Real Estate And Accumulated Depreciation Construction Year 1999      
Real Estate And Accumulated Depreciation Year Of Acquisition 1999      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 300 Berwyn Park [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,206      
Buildings and Improvements 13,422      
Net Improvements (Retirements) Since Acquisition 3,119      
Carrying Amount at Which Carried [Abstract]        
Land 2,206      
Buildings and Improvements 16,541      
Total 18,747      
Accumulated Depreciation at December 31, 2017 $ 9,112      
Real Estate And Accumulated Depreciation Construction Year 1989      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 1050 Westlakes Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,611      
Buildings and Improvements 10,445      
Net Improvements (Retirements) Since Acquisition 2,511      
Carrying Amount at Which Carried [Abstract]        
Land 2,611      
Buildings and Improvements 12,956      
Total 15,567      
Accumulated Depreciation at December 31, 2017 $ 5,622      
Real Estate And Accumulated Depreciation Construction Year 1984      
Real Estate And Accumulated Depreciation Year Of Acquisition 1999      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 1200 Swedesford Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,595      
Buildings and Improvements 11,809      
Net Improvements (Retirements) Since Acquisition 3,439      
Carrying Amount at Which Carried [Abstract]        
Land 2,595      
Buildings and Improvements 15,248      
Total 17,843      
Accumulated Depreciation at December 31, 2017 $ 8,739      
Real Estate And Accumulated Depreciation Construction Year 1994      
Real Estate And Accumulated Depreciation Year Of Acquisition 2001      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 200 Berwyn Park [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,533      
Buildings and Improvements 9,460      
Net Improvements (Retirements) Since Acquisition 2,042      
Carrying Amount at Which Carried [Abstract]        
Land 1,533      
Buildings and Improvements 11,502      
Total 13,035      
Accumulated Depreciation at December 31, 2017 $ 6,235      
Real Estate And Accumulated Depreciation Construction Year 1987      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 1180 Swedesford Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,086      
Buildings and Improvements 8,342      
Net Improvements (Retirements) Since Acquisition 3,110      
Carrying Amount at Which Carried [Abstract]        
Land 2,086      
Buildings and Improvements 11,452      
Total 13,538      
Accumulated Depreciation at December 31, 2017 $ 4,962      
Real Estate And Accumulated Depreciation Construction Year 1987      
Real Estate And Accumulated Depreciation Year Of Acquisition 2001      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 100 Berwyn Park [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,180      
Buildings and Improvements 7,290      
Net Improvements (Retirements) Since Acquisition 1,404      
Carrying Amount at Which Carried [Abstract]        
Land 1,180      
Buildings and Improvements 8,694      
Total 9,874      
Accumulated Depreciation at December 31, 2017 $ 4,643      
Real Estate And Accumulated Depreciation Construction Year 1986      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 1160 Swedesford Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,781      
Buildings and Improvements 7,124      
Net Improvements (Retirements) Since Acquisition 6,635      
Carrying Amount at Which Carried [Abstract]        
Land 2,045      
Buildings and Improvements 13,495      
Total 15,540      
Accumulated Depreciation at December 31, 2017 $ 5,036      
Real Estate And Accumulated Depreciation Construction Year 1986      
Real Estate And Accumulated Depreciation Year Of Acquisition 2001      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 1100 Cassett Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,695      
Buildings and Improvements 6,779      
Net Improvements (Retirements) Since Acquisition 1,271      
Carrying Amount at Which Carried [Abstract]        
Land 1,695      
Buildings and Improvements 8,050      
Total 9,745      
Accumulated Depreciation at December 31, 2017 $ 3,419      
Real Estate And Accumulated Depreciation Construction Year 1997      
Real Estate And Accumulated Depreciation Year Of Acquisition 2001      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | Six Tower Bridge (181 Washington Street) [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 6,927      
Buildings and Improvements 14,722      
Net Improvements (Retirements) Since Acquisition 1,762      
Carrying Amount at Which Carried [Abstract]        
Land 6,237      
Buildings and Improvements 17,174      
Total 23,411      
Accumulated Depreciation at December 31, 2017 $ 2,623      
Real Estate And Accumulated Depreciation Construction Year 1999      
Real Estate And Accumulated Depreciation Year Of Acquisition 2013      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 52 Swedesford Square [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 4,241      
Buildings and Improvements 16,579      
Net Improvements (Retirements) Since Acquisition 5,340      
Carrying Amount at Which Carried [Abstract]        
Land 4,241      
Buildings and Improvements 21,919      
Total 26,160      
Accumulated Depreciation at December 31, 2017 $ 9,736      
Real Estate And Accumulated Depreciation Construction Year 1988      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 640 Freedom Business Center [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 4,222      
Buildings and Improvements 16,891      
Net Improvements (Retirements) Since Acquisition 3,934      
Carrying Amount at Which Carried [Abstract]        
Land 4,222      
Buildings and Improvements 20,825      
Total 25,047      
Accumulated Depreciation at December 31, 2017 $ 10,685      
Real Estate And Accumulated Depreciation Construction Year 1991      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 620 Freedom Business Center [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,770      
Buildings and Improvements 11,014      
Net Improvements (Retirements) Since Acquisition 1,444      
Carrying Amount at Which Carried [Abstract]        
Land 2,770      
Buildings and Improvements 12,458      
Total 15,228      
Accumulated Depreciation at December 31, 2017 $ 6,595      
Real Estate And Accumulated Depreciation Construction Year 1986      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 1000 First Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,772      
Buildings and Improvements 10,936      
Net Improvements (Retirements) Since Acquisition 3,670      
Carrying Amount at Which Carried [Abstract]        
Land 2,772      
Buildings and Improvements 14,606      
Total 17,378      
Accumulated Depreciation at December 31, 2017 $ 6,600      
Real Estate And Accumulated Depreciation Construction Year 1980      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 1060 First Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,712      
Buildings and Improvements 10,953      
Net Improvements (Retirements) Since Acquisition 4,289      
Carrying Amount at Which Carried [Abstract]        
Land 2,712      
Buildings and Improvements 15,242      
Total 17,954      
Accumulated Depreciation at December 31, 2017 $ 7,384      
Real Estate And Accumulated Depreciation Construction Year 1987      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 630 Freedom Business Center [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,773      
Buildings and Improvements 11,144      
Net Improvements (Retirements) Since Acquisition 3,340      
Carrying Amount at Which Carried [Abstract]        
Land 2,773      
Buildings and Improvements 14,484      
Total 17,257      
Accumulated Depreciation at December 31, 2017 $ 6,850      
Real Estate And Accumulated Depreciation Construction Year 1989      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 1020 First Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,168      
Buildings and Improvements 8,576      
Net Improvements (Retirements) Since Acquisition 3,939      
Carrying Amount at Which Carried [Abstract]        
Land 2,168      
Buildings and Improvements 12,515      
Total 14,683      
Accumulated Depreciation at December 31, 2017 $ 5,513      
Real Estate And Accumulated Depreciation Construction Year 1984      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 1040 First Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,860      
Buildings and Improvements 11,282      
Net Improvements (Retirements) Since Acquisition 5,037      
Carrying Amount at Which Carried [Abstract]        
Land 2,860      
Buildings and Improvements 16,319      
Total 19,179      
Accumulated Depreciation at December 31, 2017 $ 7,320      
Real Estate And Accumulated Depreciation Construction Year 1985      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 610 Freedom Business Center [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,017      
Buildings and Improvements 8,070      
Net Improvements (Retirements) Since Acquisition 2,895      
Carrying Amount at Which Carried [Abstract]        
Land 2,017      
Buildings and Improvements 10,965      
Total 12,982      
Accumulated Depreciation at December 31, 2017 $ 5,199      
Real Estate And Accumulated Depreciation Construction Year 1985      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 650 Park Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,916      
Buildings and Improvements 4,378      
Net Improvements (Retirements) Since Acquisition 1,569      
Carrying Amount at Which Carried [Abstract]        
Land 1,916      
Buildings and Improvements 5,947      
Total 7,863      
Accumulated Depreciation at December 31, 2017 $ 3,401      
Real Estate And Accumulated Depreciation Construction Year 1968      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 600 Park Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,012      
Buildings and Improvements 4,048      
Net Improvements (Retirements) Since Acquisition 385      
Carrying Amount at Which Carried [Abstract]        
Land 1,012      
Buildings and Improvements 4,433      
Total 5,445      
Accumulated Depreciation at December 31, 2017 $ 2,406      
Real Estate And Accumulated Depreciation Construction Year 1964      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 933 First Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 3,127      
Buildings and Improvements 20,794      
Net Improvements (Retirements) Since Acquisition (1,301)      
Carrying Amount at Which Carried [Abstract]        
Land 3,127      
Buildings and Improvements 19,493      
Total 22,620      
Accumulated Depreciation at December 31, 2017 $ 482      
Real Estate And Accumulated Depreciation Construction Year 2017      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 401 Plymouth Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 6,199      
Buildings and Improvements 16,131      
Net Improvements (Retirements) Since Acquisition 16,905      
Carrying Amount at Which Carried [Abstract]        
Land 6,199      
Buildings and Improvements 33,036      
Total 39,235      
Accumulated Depreciation at December 31, 2017 $ 13,795      
Real Estate And Accumulated Depreciation Construction Year 2001      
Real Estate And Accumulated Depreciation Year Of Acquisition 2000      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | Metroplex (4000 Chemical Road) [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 4,373      
Buildings and Improvements 24,546      
Net Improvements (Retirements) Since Acquisition 679      
Carrying Amount at Which Carried [Abstract]        
Land 4,373      
Buildings and Improvements 25,225      
Total 29,598      
Accumulated Depreciation at December 31, 2017 $ 6,437      
Real Estate And Accumulated Depreciation Construction Year 2007      
Real Estate And Accumulated Depreciation Year Of Acquisition 2001      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 610 West Germantown Pike [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 3,651      
Buildings and Improvements 14,514      
Net Improvements (Retirements) Since Acquisition 3,172      
Carrying Amount at Which Carried [Abstract]        
Land 3,651      
Buildings and Improvements 17,686      
Total 21,337      
Accumulated Depreciation at December 31, 2017 $ 7,309      
Real Estate And Accumulated Depreciation Construction Year 1987      
Real Estate And Accumulated Depreciation Year Of Acquisition 2002      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 600 West Germantown Pike [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 3,652      
Buildings and Improvements 15,288      
Net Improvements (Retirements) Since Acquisition 2,251      
Carrying Amount at Which Carried [Abstract]        
Land 3,652      
Buildings and Improvements 17,539      
Total 21,191      
Accumulated Depreciation at December 31, 2017 $ 6,877      
Real Estate And Accumulated Depreciation Construction Year 1986      
Real Estate And Accumulated Depreciation Year Of Acquisition 2002      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 630 West Germantown Pike [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 3,558      
Buildings and Improvements 14,743      
Net Improvements (Retirements) Since Acquisition 2,172      
Carrying Amount at Which Carried [Abstract]        
Land 3,558      
Buildings and Improvements 16,915      
Total 20,473      
Accumulated Depreciation at December 31, 2017 $ 6,645      
Real Estate And Accumulated Depreciation Construction Year 1988      
Real Estate And Accumulated Depreciation Year Of Acquisition 2002      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 620 West Germantown Pike [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 3,572      
Buildings and Improvements 14,435      
Net Improvements (Retirements) Since Acquisition 1,199      
Carrying Amount at Which Carried [Abstract]        
Land 3,572      
Buildings and Improvements 15,634      
Total 19,206      
Accumulated Depreciation at December 31, 2017 $ 6,262      
Real Estate And Accumulated Depreciation Construction Year 1990      
Real Estate And Accumulated Depreciation Year Of Acquisition 2002      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 660 West Germantown Pike [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 3,694      
Buildings and Improvements 5,487      
Net Improvements (Retirements) Since Acquisition 20,715      
Carrying Amount at Which Carried [Abstract]        
Land 5,405      
Buildings and Improvements 24,491      
Total 29,896      
Accumulated Depreciation at December 31, 2017 $ 4,429      
Real Estate And Accumulated Depreciation Construction Year 1987      
Real Estate And Accumulated Depreciation Year Of Acquisition 2012      
Depreciable Life (Years) 30 years      
PENNSYLVANIA SUBURBS [Member] | 351 Plymouth Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,043      
Buildings and Improvements 555      
Net Improvements (Retirements) Since Acquisition 0      
Carrying Amount at Which Carried [Abstract]        
Land 1,043      
Buildings and Improvements 555      
Total 1,598      
Accumulated Depreciation at December 31, 2017 $ 177      
Real Estate And Accumulated Depreciation Year Of Acquisition 2000      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 150 Radnor Chester Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 11,925      
Buildings and Improvements 36,986      
Net Improvements (Retirements) Since Acquisition 12,659      
Carrying Amount at Which Carried [Abstract]        
Land 11,897      
Buildings and Improvements 49,673      
Total 61,570      
Accumulated Depreciation at December 31, 2017 $ 21,400      
Real Estate And Accumulated Depreciation Construction Year 1983      
Real Estate And Accumulated Depreciation Year Of Acquisition 2004      
Depreciable Life (Years) 29 years      
PENNSYLVANIA SUBURBS [Member] | One Radnor Corporate Center [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 7,323      
Buildings and Improvements 28,613      
Net Improvements (Retirements) Since Acquisition 22,440      
Carrying Amount at Which Carried [Abstract]        
Land 7,323      
Buildings and Improvements 51,053      
Total 58,376      
Accumulated Depreciation at December 31, 2017 $ 26,157      
Real Estate And Accumulated Depreciation Construction Year 1998      
Real Estate And Accumulated Depreciation Year Of Acquisition 2004      
Depreciable Life (Years) 29 years      
PENNSYLVANIA SUBURBS [Member] | 201 King of Prussia Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 8,956      
Buildings and Improvements 29,811      
Net Improvements (Retirements) Since Acquisition 3,800      
Carrying Amount at Which Carried [Abstract]        
Land 8,949      
Buildings and Improvements 33,618      
Total 42,567      
Accumulated Depreciation at December 31, 2017 $ 18,855      
Real Estate And Accumulated Depreciation Construction Year 2001      
Real Estate And Accumulated Depreciation Year Of Acquisition 2004      
Depreciable Life (Years) 25 years      
PENNSYLVANIA SUBURBS [Member] | 555 Lancaster Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 8,014      
Buildings and Improvements 16,508      
Net Improvements (Retirements) Since Acquisition 16,861      
Carrying Amount at Which Carried [Abstract]        
Land 8,609      
Buildings and Improvements 32,774      
Total 41,383      
Accumulated Depreciation at December 31, 2017 $ 15,517      
Real Estate And Accumulated Depreciation Construction Year 1973      
Real Estate And Accumulated Depreciation Year Of Acquisition 2004      
Depreciable Life (Years) 24 years      
PENNSYLVANIA SUBURBS [Member] | Four Radnor Corporate Center [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 5,406      
Buildings and Improvements 21,390      
Net Improvements (Retirements) Since Acquisition 12,190      
Carrying Amount at Which Carried [Abstract]        
Land 5,705      
Buildings and Improvements 33,281      
Total 38,986      
Accumulated Depreciation at December 31, 2017 $ 13,192      
Real Estate And Accumulated Depreciation Construction Year 1995      
Real Estate And Accumulated Depreciation Year Of Acquisition 2004      
Depreciable Life (Years) 30 years      
PENNSYLVANIA SUBURBS [Member] | Five Radnor Corporate Center [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 6,506      
Buildings and Improvements 25,525      
Net Improvements (Retirements) Since Acquisition 4,373      
Carrying Amount at Which Carried [Abstract]        
Land 6,578      
Buildings and Improvements 29,826      
Total 36,404      
Accumulated Depreciation at December 31, 2017 $ 10,451      
Real Estate And Accumulated Depreciation Construction Year 1998      
Real Estate And Accumulated Depreciation Year Of Acquisition 2004      
Depreciable Life (Years) 38 years      
PENNSYLVANIA SUBURBS [Member] | Three Radnor Corporate Center [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 4,773      
Buildings and Improvements 17,961      
Net Improvements (Retirements) Since Acquisition 2,790      
Carrying Amount at Which Carried [Abstract]        
Land 4,791      
Buildings and Improvements 20,733      
Total 25,524      
Accumulated Depreciation at December 31, 2017 $ 10,342      
Real Estate And Accumulated Depreciation Construction Year 1998      
Real Estate And Accumulated Depreciation Year Of Acquisition 2004      
Depreciable Life (Years) 29 years      
PENNSYLVANIA SUBURBS [Member] | Two Radnor Corporate Center [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 3,937      
Buildings and Improvements 15,484      
Net Improvements (Retirements) Since Acquisition 3,440      
Carrying Amount at Which Carried [Abstract]        
Land 3,942      
Buildings and Improvements 18,919      
Total 22,861      
Accumulated Depreciation at December 31, 2017 $ 8,458      
Real Estate And Accumulated Depreciation Construction Year 1998      
Real Estate And Accumulated Depreciation Year Of Acquisition 2004      
Depreciable Life (Years) 29 years      
PENNSYLVANIA SUBURBS [Member] | 130 Radnor Chester Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,573      
Buildings and Improvements 8,338      
Net Improvements (Retirements) Since Acquisition 3,460      
Carrying Amount at Which Carried [Abstract]        
Land 2,567      
Buildings and Improvements 11,804      
Total 14,371      
Accumulated Depreciation at December 31, 2017 $ 5,948      
Real Estate And Accumulated Depreciation Construction Year 1983      
Real Estate And Accumulated Depreciation Year Of Acquisition 2004      
Depreciable Life (Years) 25 years      
PENNSYLVANIA SUBURBS [Member] | 170 Radnor Chester Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,514      
Buildings and Improvements 8,147      
Net Improvements (Retirements) Since Acquisition 2,132      
Carrying Amount at Which Carried [Abstract]        
Land 2,509      
Buildings and Improvements 10,284      
Total 12,793      
Accumulated Depreciation at December 31, 2017 $ 3,739      
Real Estate And Accumulated Depreciation Construction Year 1983      
Real Estate And Accumulated Depreciation Year Of Acquisition 2004      
Depreciable Life (Years) 25 years      
PENNSYLVANIA SUBURBS [Member] | 200 N. Radnor Chester Rd. [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 3,366      
Buildings and Improvements 0      
Net Improvements (Retirements) Since Acquisition 3,572      
Carrying Amount at Which Carried [Abstract]        
Land 3,366      
Buildings and Improvements 3,572      
Total 6,938      
Accumulated Depreciation at December 31, 2017 $ 503      
Real Estate And Accumulated Depreciation Construction Year 2014      
Real Estate And Accumulated Depreciation Year Of Acquisition 2005      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 101 West Elm Street [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 6,251      
Buildings and Improvements 25,209      
Net Improvements (Retirements) Since Acquisition 2,872      
Carrying Amount at Which Carried [Abstract]        
Land 6,251      
Buildings and Improvements 28,081      
Total 34,332      
Accumulated Depreciation at December 31, 2017 $ 8,968      
Real Estate And Accumulated Depreciation Construction Year 1999      
Real Estate And Accumulated Depreciation Year Of Acquisition 2005      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 1 West Elm Street [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 3,557      
Buildings and Improvements 14,249      
Net Improvements (Retirements) Since Acquisition 3,151      
Carrying Amount at Which Carried [Abstract]        
Land 3,557      
Buildings and Improvements 17,400      
Total 20,957      
Accumulated Depreciation at December 31, 2017 $ 5,290      
Real Estate And Accumulated Depreciation Construction Year 1999      
Real Estate And Accumulated Depreciation Year Of Acquisition 2005      
Depreciable Life (Years) 40 years      
Philadelphia CBD [Member] | Cira Centre (2929 Arch Street) [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 0      
Buildings and Improvements 208,570      
Net Improvements (Retirements) Since Acquisition (6,931)      
Carrying Amount at Which Carried [Abstract]        
Land 12,586      
Buildings and Improvements 189,053      
Total 201,639      
Accumulated Depreciation at December 31, 2017 $ 68,099      
Real Estate And Accumulated Depreciation Construction Year 2005      
Depreciable Life (Years) 40 years      
Philadelphia CBD [Member] | Three Logan Square (1717 Arch Street) [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 0      
Buildings and Improvements 98,188      
Net Improvements (Retirements) Since Acquisition 61,060      
Carrying Amount at Which Carried [Abstract]        
Land 25,195      
Buildings and Improvements 134,053      
Total 159,248      
Accumulated Depreciation at December 31, 2017 $ 31,079      
Real Estate And Accumulated Depreciation Construction Year 1990      
Real Estate And Accumulated Depreciation Year Of Acquisition 2010      
Depreciable Life (Years) 40 years      
Philadelphia CBD [Member] | Two Commerce Square (2001 Market Street) [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 112,000      
Initial Cost [Abstract]        
Land 15,323      
Buildings and Improvements 120,200      
Net Improvements (Retirements) Since Acquisition 21,892      
Carrying Amount at Which Carried [Abstract]        
Land 15,323      
Buildings and Improvements 142,092      
Total 157,415      
Accumulated Depreciation at December 31, 2017 $ 17,086      
Real Estate And Accumulated Depreciation Construction Year 1992      
Real Estate And Accumulated Depreciation Year Of Acquisition 2013      
Depreciable Life (Years) 40 years      
Philadelphia CBD [Member] | One Logan Square (130 North 18th Street) [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 14,496      
Buildings and Improvements 107,736      
Net Improvements (Retirements) Since Acquisition 28,938      
Carrying Amount at Which Carried [Abstract]        
Land 14,473      
Buildings and Improvements 136,697      
Total 151,170      
Accumulated Depreciation at December 31, 2017 $ 52,506      
Real Estate And Accumulated Depreciation Construction Year 1998      
Real Estate And Accumulated Depreciation Year Of Acquisition 2004      
Depreciable Life (Years) 34 years      
Philadelphia CBD [Member] | Two Logan Square (100 North 18th Street) [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 84,440      
Initial Cost [Abstract]        
Land 16,066      
Buildings and Improvements 100,255      
Net Improvements (Retirements) Since Acquisition 19,733      
Carrying Amount at Which Carried [Abstract]        
Land 16,066      
Buildings and Improvements 119,988      
Total 136,054      
Accumulated Depreciation at December 31, 2017 $ 42,317      
Real Estate And Accumulated Depreciation Construction Year 1988      
Real Estate And Accumulated Depreciation Year Of Acquisition 2004      
Depreciable Life (Years) 36 years      
Philadelphia CBD [Member] | One Commerce Square (2005 Market Street) [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 123,667      
Initial Cost [Abstract]        
Land 15,161      
Buildings and Improvements 105,021      
Net Improvements (Retirements) Since Acquisition 22,924      
Carrying Amount at Which Carried [Abstract]        
Land 15,160      
Buildings and Improvements 127,946      
Total 143,106      
Accumulated Depreciation at December 31, 2017 $ 15,828      
Real Estate And Accumulated Depreciation Construction Year 1987      
Real Estate And Accumulated Depreciation Year Of Acquisition 2013      
Depreciable Life (Years) 40 years      
Philadelphia CBD [Member] | Cira Centre South Garage [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 0      
Buildings and Improvements 76,008      
Net Improvements (Retirements) Since Acquisition 8,114      
Carrying Amount at Which Carried [Abstract]        
Land 0      
Buildings and Improvements 84,122      
Total 84,122      
Accumulated Depreciation at December 31, 2017 $ 14,732      
Real Estate And Accumulated Depreciation Construction Year 2010      
Depreciable Life (Years) 40 years      
Philadelphia CBD [Member] | 1900 Market [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 7,768      
Buildings and Improvements 17,263      
Net Improvements (Retirements) Since Acquisition 46,556      
Carrying Amount at Which Carried [Abstract]        
Land 7,768      
Buildings and Improvements 63,819      
Total 71,587      
Accumulated Depreciation at December 31, 2017 $ 7,484      
Real Estate And Accumulated Depreciation Construction Year 1981      
Real Estate And Accumulated Depreciation Year Of Acquisition 2012      
Depreciable Life (Years) 30 years      
Philadelphia CBD [Member] | 3020 Market Street [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 0      
Buildings and Improvements 21,417      
Net Improvements (Retirements) Since Acquisition 7,603      
Carrying Amount at Which Carried [Abstract]        
Land 0      
Buildings and Improvements 29,020      
Total 29,020      
Accumulated Depreciation at December 31, 2017 $ 7,511      
Real Estate And Accumulated Depreciation Construction Year 1959      
Real Estate And Accumulated Depreciation Year Of Acquisition 2011      
Depreciable Life (Years) 26 years      
Philadelphia CBD [Member] | The Lift at Juniper Street (101 - 103 Juniper Street) [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 0      
Buildings and Improvements 14,401      
Net Improvements (Retirements) Since Acquisition (3,329)      
Carrying Amount at Which Carried [Abstract]        
Land 478      
Buildings and Improvements 10,594      
Total 11,072      
Accumulated Depreciation at December 31, 2017 $ 2,803      
Real Estate And Accumulated Depreciation Construction Year 2010      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 40 years      
Philadelphia CBD [Member] | 618-634 Market Street [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 13,365      
Buildings and Improvements 5,791      
Net Improvements (Retirements) Since Acquisition 490      
Carrying Amount at Which Carried [Abstract]        
Land 13,365      
Buildings and Improvements 6,281      
Total 19,646      
Accumulated Depreciation at December 31, 2017 $ 3,258      
Real Estate And Accumulated Depreciation Construction Year 1966      
Real Estate And Accumulated Depreciation Year Of Acquisition 2015      
Depreciable Life (Years) 5 years      
Philadelphia CBD [Member] | FMC Tower at Cira Centre South (2929 Walnut Street) [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 0      
Buildings and Improvements 400,294      
Net Improvements (Retirements) Since Acquisition (7,976)      
Carrying Amount at Which Carried [Abstract]        
Land 0      
Buildings and Improvements 392,318      
Total 392,318      
Accumulated Depreciation at December 31, 2017 $ 16,218      
Real Estate And Accumulated Depreciation Construction Year 2016      
Depreciable Life (Years) 40 years      
Philadelphia CBD [Member] | 2100 Market Street [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 18,827      
Buildings and Improvements 0      
Net Improvements (Retirements) Since Acquisition 5,494      
Carrying Amount at Which Carried [Abstract]        
Land 18,854      
Buildings and Improvements 5,467      
Total 24,321      
Accumulated Depreciation at December 31, 2017 $ 121      
Real Estate And Accumulated Depreciation Year Of Acquisition 2015      
Philadelphia CBD [Member] | 3000 Market Street [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 18,924      
Buildings and Improvements 13,080      
Net Improvements (Retirements) Since Acquisition 8      
Carrying Amount at Which Carried [Abstract]        
Land 18,924      
Buildings and Improvements 13,088      
Total 32,012      
Accumulated Depreciation at December 31, 2017 $ 569      
Real Estate And Accumulated Depreciation Construction Year 1937      
Real Estate And Accumulated Depreciation Year Of Acquisition 2017      
Depreciable Life (Years) 12 years      
Philadelphia CBD [Member] | One Drexel Plaza [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 0      
Buildings and Improvements 24,377      
Net Improvements (Retirements) Since Acquisition 6,206      
Carrying Amount at Which Carried [Abstract]        
Land 0      
Buildings and Improvements 30,583      
Total 30,583      
Accumulated Depreciation at December 31, 2017 $ 194      
Real Estate And Accumulated Depreciation Construction Year 1953      
Real Estate And Accumulated Depreciation Year Of Acquisition 2017      
Depreciable Life (Years) 40 years      
Metropolitan DC [Member] | 6600 Rockledge Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 0      
Buildings and Improvements 37,421      
Net Improvements (Retirements) Since Acquisition 10,841      
Carrying Amount at Which Carried [Abstract]        
Land 0      
Buildings and Improvements 48,262      
Total 48,262      
Accumulated Depreciation at December 31, 2017 $ 13,299      
Real Estate And Accumulated Depreciation Construction Year 1981      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 50 years      
Metropolitan DC [Member] | 2340 Dulles Corner Boulevard [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 16,345      
Buildings and Improvements 65,379      
Net Improvements (Retirements) Since Acquisition 3,710      
Carrying Amount at Which Carried [Abstract]        
Land 16,129      
Buildings and Improvements 69,305      
Total 85,434      
Accumulated Depreciation at December 31, 2017 $ 21,809      
Real Estate And Accumulated Depreciation Construction Year 1987      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 40 years      
Metropolitan DC [Member] | 2291 Wood Oak Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 8,243      
Buildings and Improvements 52,413      
Net Improvements (Retirements) Since Acquisition 12,549      
Carrying Amount at Which Carried [Abstract]        
Land 8,782      
Buildings and Improvements 64,423      
Total 73,205      
Accumulated Depreciation at December 31, 2017 $ 19,549      
Real Estate And Accumulated Depreciation Construction Year 1999      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 55 years      
Metropolitan DC [Member] | 2251 Corporate Park Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 11,472      
Buildings and Improvements 45,893      
Net Improvements (Retirements) Since Acquisition 3,412      
Carrying Amount at Which Carried [Abstract]        
Land 11,472      
Buildings and Improvements 49,305      
Total 60,777      
Accumulated Depreciation at December 31, 2017 $ 14,191      
Real Estate And Accumulated Depreciation Construction Year 2000      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 40 years      
Metropolitan DC [Member] | 2355 Dulles Corner Boulevard [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 10,365      
Buildings and Improvements 43,876      
Net Improvements (Retirements) Since Acquisition 7,299      
Carrying Amount at Which Carried [Abstract]        
Land 10,365      
Buildings and Improvements 51,175      
Total 61,540      
Accumulated Depreciation at December 31, 2017 $ 16,202      
Real Estate And Accumulated Depreciation Construction Year 1988      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 40 years      
Metropolitan DC [Member] | 2411 Dulles Corner Park [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 7,279      
Buildings and Improvements 46,340      
Net Improvements (Retirements) Since Acquisition 17,626      
Carrying Amount at Which Carried [Abstract]        
Land 7,417      
Buildings and Improvements 63,828      
Total 71,245      
Accumulated Depreciation at December 31, 2017 $ 16,736      
Real Estate And Accumulated Depreciation Construction Year 1990      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 50 years      
Metropolitan DC [Member] | 13880 Dulles Corner Lane [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 7,236      
Buildings and Improvements 39,213      
Net Improvements (Retirements) Since Acquisition 5,904      
Carrying Amount at Which Carried [Abstract]        
Land 7,373      
Buildings and Improvements 44,980      
Total 52,353      
Accumulated Depreciation at December 31, 2017 $ 11,525      
Real Estate And Accumulated Depreciation Construction Year 1997      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 55 years      
Metropolitan DC [Member] | 2121 Cooperative Way [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 5,598      
Buildings and Improvements 38,639      
Net Improvements (Retirements) Since Acquisition 3,186      
Carrying Amount at Which Carried [Abstract]        
Land 5,795      
Buildings and Improvements 41,628      
Total 47,423      
Accumulated Depreciation at December 31, 2017 $ 11,035      
Real Estate And Accumulated Depreciation Construction Year 2000      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 54 years      
Metropolitan DC [Member] | 2201 Cooperative Way [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 4,809      
Buildings and Improvements 34,093      
Net Improvements (Retirements) Since Acquisition 6,018      
Carrying Amount at Which Carried [Abstract]        
Land 4,809      
Buildings and Improvements 40,111      
Total 44,920      
Accumulated Depreciation at December 31, 2017 $ 10,205      
Real Estate And Accumulated Depreciation Construction Year 1990      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 54 years      
Metropolitan DC [Member] | 13825 Sunrise Valley Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 3,794      
Buildings and Improvements 19,365      
Net Improvements (Retirements) Since Acquisition 1,454      
Carrying Amount at Which Carried [Abstract]        
Land 3,866      
Buildings and Improvements 20,747      
Total 24,613      
Accumulated Depreciation at December 31, 2017 $ 6,202      
Real Estate And Accumulated Depreciation Construction Year 1989      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 46 years      
Metropolitan DC [Member] | 1676 International Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 18,437      
Buildings and Improvements 97,538      
Net Improvements (Retirements) Since Acquisition 3,744      
Carrying Amount at Which Carried [Abstract]        
Land 18,785      
Buildings and Improvements 100,934      
Total 119,719      
Accumulated Depreciation at December 31, 2017 $ 23,545      
Real Estate And Accumulated Depreciation Construction Year 1999      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 55 years      
Metropolitan DC [Member] | 8260 Greensboro Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 7,952      
Buildings and Improvements 33,964      
Net Improvements (Retirements) Since Acquisition 3,184      
Carrying Amount at Which Carried [Abstract]        
Land 8,102      
Buildings and Improvements 36,998      
Total 45,100      
Accumulated Depreciation at December 31, 2017 $ 10,182      
Real Estate And Accumulated Depreciation Construction Year 1980      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 52 years      
Metropolitan DC [Member] | 2273 Research Boulevard [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 5,167      
Buildings and Improvements 31,110      
Net Improvements (Retirements) Since Acquisition 4,098      
Carrying Amount at Which Carried [Abstract]        
Land 5,237      
Buildings and Improvements 35,138      
Total 40,375      
Accumulated Depreciation at December 31, 2017 $ 9,332      
Real Estate And Accumulated Depreciation Construction Year 1999      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 45 years      
Metropolitan DC [Member] | 2275 Research Boulevard [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 5,059      
Buildings and Improvements 29,668      
Net Improvements (Retirements) Since Acquisition 8,093      
Carrying Amount at Which Carried [Abstract]        
Land 5,154      
Buildings and Improvements 37,666      
Total 42,820      
Accumulated Depreciation at December 31, 2017 $ 10,989      
Real Estate And Accumulated Depreciation Construction Year 1990      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 45 years      
Metropolitan DC [Member] | 2277 Research Boulevard [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 4,649      
Buildings and Improvements 26,952      
Net Improvements (Retirements) Since Acquisition 18,867      
Carrying Amount at Which Carried [Abstract]        
Land 4,733      
Buildings and Improvements 45,735      
Total 50,468      
Accumulated Depreciation at December 31, 2017 $ 12,369      
Real Estate And Accumulated Depreciation Construction Year 1986      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 45 years      
Metropolitan DC [Member] | 1900 Gallows Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 7,797      
Buildings and Improvements 47,817      
Net Improvements (Retirements) Since Acquisition 12,333      
Carrying Amount at Which Carried [Abstract]        
Land 7,944      
Buildings and Improvements 60,003      
Total 67,947      
Accumulated Depreciation at December 31, 2017 $ 17,697      
Real Estate And Accumulated Depreciation Construction Year 1989      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 52 years      
Metropolitan DC [Member] | 8521 Leesburg Pike [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 4,316      
Buildings and Improvements 30,885      
Net Improvements (Retirements) Since Acquisition 6,846      
Carrying Amount at Which Carried [Abstract]        
Land 4,397      
Buildings and Improvements 37,650      
Total 42,047      
Accumulated Depreciation at December 31, 2017 $ 9,854      
Real Estate And Accumulated Depreciation Construction Year 1984      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 51 years      
Austin, Texas [Member] | 11501 Burnet Road - Building 1 [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 3,755      
Buildings and Improvements 22,702      
Net Improvements (Retirements) Since Acquisition 130      
Carrying Amount at Which Carried [Abstract]        
Land 3,755      
Buildings and Improvements 22,832      
Total 26,587      
Accumulated Depreciation at December 31, 2017 $ 1,783      
Real Estate And Accumulated Depreciation Construction Year 1991      
Real Estate And Accumulated Depreciation Year Of Acquisition 2015      
Depreciable Life (Years) 35 years      
Austin, Texas [Member] | 11501 Burnet Road - Building 2 [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,732      
Buildings and Improvements 16,305      
Net Improvements (Retirements) Since Acquisition 1,572      
Carrying Amount at Which Carried [Abstract]        
Land 2,732      
Buildings and Improvements 17,877      
Total 20,609      
Accumulated Depreciation at December 31, 2017 $ 1,606      
Real Estate And Accumulated Depreciation Construction Year 1991      
Real Estate And Accumulated Depreciation Year Of Acquisition 2015      
Depreciable Life (Years) 35 years      
Austin, Texas [Member] | 11501 Burnet Road - Building 3 [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 3,688      
Buildings and Improvements 22,348      
Net Improvements (Retirements) Since Acquisition 128      
Carrying Amount at Which Carried [Abstract]        
Land 3,688      
Buildings and Improvements 22,476      
Total 26,164      
Accumulated Depreciation at December 31, 2017 $ 1,755      
Real Estate And Accumulated Depreciation Construction Year 1991      
Real Estate And Accumulated Depreciation Year Of Acquisition 2015      
Depreciable Life (Years) 35 years      
Austin, Texas [Member] | 11501 Burnet Road - Building 4 [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,614      
Buildings and Improvements 15,740      
Net Improvements (Retirements) Since Acquisition 90      
Carrying Amount at Which Carried [Abstract]        
Land 2,614      
Buildings and Improvements 15,830      
Total 18,444      
Accumulated Depreciation at December 31, 2017 $ 1,236      
Real Estate And Accumulated Depreciation Construction Year 1991      
Real Estate And Accumulated Depreciation Year Of Acquisition 2015      
Depreciable Life (Years) 35 years      
Austin, Texas [Member] | 11501 Burnet Road - Building 5 [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 3,689      
Buildings and Improvements 22,354      
Net Improvements (Retirements) Since Acquisition 128      
Carrying Amount at Which Carried [Abstract]        
Land 3,689      
Buildings and Improvements 22,482      
Total 26,171      
Accumulated Depreciation at December 31, 2017 $ 1,755      
Real Estate And Accumulated Depreciation Construction Year 1991      
Real Estate And Accumulated Depreciation Year Of Acquisition 2015      
Depreciable Life (Years) 35 years      
Austin, Texas [Member] | 11501 Burnet Road - Building 6 [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,676      
Buildings and Improvements 15,972      
Net Improvements (Retirements) Since Acquisition (1,835)      
Carrying Amount at Which Carried [Abstract]        
Land 2,676      
Buildings and Improvements 14,137      
Total 16,813      
Accumulated Depreciation at December 31, 2017 $ 121      
Real Estate And Accumulated Depreciation Construction Year 1991      
Real Estate And Accumulated Depreciation Year Of Acquisition 2015      
Depreciable Life (Years) 40 years      
Austin, Texas [Member] | 11501 Burnet Road - Building 8 [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,400      
Buildings and Improvements 7,422      
Net Improvements (Retirements) Since Acquisition 104      
Carrying Amount at Which Carried [Abstract]        
Land 1,400      
Buildings and Improvements 7,526      
Total 8,926      
Accumulated Depreciation at December 31, 2017 $ 591      
Real Estate And Accumulated Depreciation Construction Year 1991      
Real Estate And Accumulated Depreciation Year Of Acquisition 2015      
Depreciable Life (Years) 35 years      
Austin, Texas [Member] | 11501 Burnet Road - Parking Garage [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 0      
Buildings and Improvements 19,826      
Net Improvements (Retirements) Since Acquisition 113      
Carrying Amount at Which Carried [Abstract]        
Land 0      
Buildings and Improvements 19,939      
Total 19,939      
Accumulated Depreciation at December 31, 2017 $ 2,070      
Real Estate And Accumulated Depreciation Construction Year 1991      
Real Estate And Accumulated Depreciation Year Of Acquisition 2015      
Depreciable Life (Years) 35 years      
Other [Member] | 20 East Clementon Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 769      
Buildings and Improvements 3,055      
Net Improvements (Retirements) Since Acquisition 506      
Carrying Amount at Which Carried [Abstract]        
Land 719      
Buildings and Improvements 3,611      
Total 4,330      
Accumulated Depreciation at December 31, 2017 $ 1,892      
Real Estate And Accumulated Depreciation Construction Year 1986      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
Other [Member] | 10 Foster Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 244      
Buildings and Improvements 971      
Net Improvements (Retirements) Since Acquisition 69      
Carrying Amount at Which Carried [Abstract]        
Land 244      
Buildings and Improvements 1,040      
Total 1,284      
Accumulated Depreciation at December 31, 2017 $ 571      
Real Estate And Accumulated Depreciation Construction Year 1983      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
Other [Member] | 7 Foster Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 231      
Buildings and Improvements 921      
Net Improvements (Retirements) Since Acquisition 17      
Carrying Amount at Which Carried [Abstract]        
Land 231      
Buildings and Improvements 938      
Total 1,169      
Accumulated Depreciation at December 31, 2017 $ 501      
Real Estate And Accumulated Depreciation Construction Year 1983      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
Other [Member] | 2 Foster Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 185      
Buildings and Improvements 730      
Net Improvements (Retirements) Since Acquisition 11      
Carrying Amount at Which Carried [Abstract]        
Land 185      
Buildings and Improvements 741      
Total 926      
Accumulated Depreciation at December 31, 2017 $ 741      
Real Estate And Accumulated Depreciation Construction Year 1974      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
Other [Member] | 4 Foster Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 183      
Buildings and Improvements 726      
Net Improvements (Retirements) Since Acquisition 6      
Carrying Amount at Which Carried [Abstract]        
Land 183      
Buildings and Improvements 732      
Total 915      
Accumulated Depreciation at December 31, 2017 $ 732      
Real Estate And Accumulated Depreciation Construction Year 1974      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
Other [Member] | 1 Foster Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 93      
Buildings and Improvements 364      
Net Improvements (Retirements) Since Acquisition 8      
Carrying Amount at Which Carried [Abstract]        
Land 93      
Buildings and Improvements 372      
Total 465      
Accumulated Depreciation at December 31, 2017 $ 372      
Real Estate And Accumulated Depreciation Construction Year 1972      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
Other [Member] | 5 U.S. Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 21      
Buildings and Improvements 81      
Net Improvements (Retirements) Since Acquisition 2      
Carrying Amount at Which Carried [Abstract]        
Land 21      
Buildings and Improvements 83      
Total 104      
Accumulated Depreciation at December 31, 2017 $ 45      
Real Estate And Accumulated Depreciation Construction Year 1987      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
Other [Member] | 5 Foster Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 9      
Buildings and Improvements 32      
Net Improvements (Retirements) Since Acquisition 26      
Carrying Amount at Which Carried [Abstract]        
Land 9      
Buildings and Improvements 58      
Total 67      
Accumulated Depreciation at December 31, 2017 $ 33      
Real Estate And Accumulated Depreciation Construction Year 1968      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
Other [Member] | Main Street-Plaza 1000 [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,732      
Buildings and Improvements 10,942      
Net Improvements (Retirements) Since Acquisition 376      
Carrying Amount at Which Carried [Abstract]        
Land 2,732      
Buildings and Improvements 11,318      
Total 14,050      
Accumulated Depreciation at December 31, 2017 $ 11,037      
Real Estate And Accumulated Depreciation Construction Year 1988      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
Other [Member] | Main Street-Piazza [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 696      
Buildings and Improvements 2,802      
Net Improvements (Retirements) Since Acquisition 3,446      
Carrying Amount at Which Carried [Abstract]        
Land 700      
Buildings and Improvements 6,244      
Total 6,944      
Accumulated Depreciation at December 31, 2017 $ 2,741      
Real Estate And Accumulated Depreciation Construction Year 1990      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
Other [Member] | Main Street-Promenade [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 532      
Buildings and Improvements 2,052      
Net Improvements (Retirements) Since Acquisition 404      
Carrying Amount at Which Carried [Abstract]        
Land 532      
Buildings and Improvements 2,456      
Total 2,988      
Accumulated Depreciation at December 31, 2017 $ 1,357      
Real Estate And Accumulated Depreciation Construction Year 1988      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
Other [Member] | 920 North King Street [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 6,141      
Buildings and Improvements 21,140      
Net Improvements (Retirements) Since Acquisition 5,003      
Carrying Amount at Which Carried [Abstract]        
Land 6,141      
Buildings and Improvements 26,143      
Total 32,284      
Accumulated Depreciation at December 31, 2017 $ 10,185      
Real Estate And Accumulated Depreciation Construction Year 1989      
Real Estate And Accumulated Depreciation Year Of Acquisition 2004      
Depreciable Life (Years) 30 years      
Other [Member] | 300 Delaware Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 6,369      
Buildings and Improvements 13,739      
Net Improvements (Retirements) Since Acquisition 2,708      
Carrying Amount at Which Carried [Abstract]        
Land 6,369      
Buildings and Improvements 16,447      
Total 22,816      
Accumulated Depreciation at December 31, 2017 $ 8,762      
Real Estate And Accumulated Depreciation Construction Year 1989      
Real Estate And Accumulated Depreciation Year Of Acquisition 2004      
Depreciable Life (Years) 23 years