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Schedule III - Real Estate and Accumulated Depreciation (Details) - USD ($)
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 323,032,000      
Initial Cost [Abstract]        
Land 453,723,000      
Buildings and Improvements 2,863,832,000      
Net Improvements (Retirements) Since Acquisition 635,764,000      
Carrying Amount at Which Carried [Abstract]        
Land 508,363,000      
Buildings and Improvements 3,444,955,000      
Total 3,953,319,000 $ 3,832,348,000 $ 3,659,886,000 $ 4,487,588,000
Accumulated Depreciation at December 31, 2018 865,462,000 $ 895,091,000 $ 885,392,000 $ 1,080,616,000
PENNSYLVANIA SUBURBS [Member] | 400 Berwyn Park [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances 0      
Initial Cost [Abstract]        
Land 2,657,000      
Buildings and Improvements 4,462,000      
Net Improvements (Retirements) Since Acquisition 12,985,000      
Carrying Amount at Which Carried [Abstract]        
Land 2,657,000      
Buildings and Improvements 17,447,000      
Total 20,104,000.00      
Accumulated Depreciation at December 31, 2018 $ 8,060,000      
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 2,206,000      
Buildings and Improvements 13,422,000      
Net Improvements (Retirements) Since Acquisition 3,331,000      
Carrying Amount at Which Carried [Abstract]        
Land 2,206,000      
Buildings and Improvements 16,753,000      
Total 18,959,000.00      
Accumulated Depreciation at December 31, 2018 $ 9,777,000      
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 2,611,000      
Buildings and Improvements 10,445,000      
Net Improvements (Retirements) Since Acquisition 2,416,000      
Carrying Amount at Which Carried [Abstract]        
Land 2,611,000      
Buildings and Improvements 12,861,000      
Total 15,472,000.00      
Accumulated Depreciation at December 31, 2018 $ 5,990,000      
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 2,595,000      
Buildings and Improvements 11,809,000      
Net Improvements (Retirements) Since Acquisition 218,000      
Carrying Amount at Which Carried [Abstract]        
Land 2,595,000      
Buildings and Improvements 12,027,000      
Total 14,622,000.00      
Accumulated Depreciation at December 31, 2018 $ 6,211,000      
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 1,533,000      
Buildings and Improvements 9,460,000      
Net Improvements (Retirements) Since Acquisition 2,074,000      
Carrying Amount at Which Carried [Abstract]        
Land 1,533,000      
Buildings and Improvements 11,534,000      
Total 13,067,000.00      
Accumulated Depreciation at December 31, 2018 $ 6,586,000      
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 2,086,000      
Buildings and Improvements 8,342,000      
Net Improvements (Retirements) Since Acquisition 3,110,000      
Carrying Amount at Which Carried [Abstract]        
Land 2,086,000      
Buildings and Improvements 11,452,000      
Total 13,538,000.00      
Accumulated Depreciation at December 31, 2018 $ 5,496,000      
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 1,180,000      
Buildings and Improvements 7,290,000      
Net Improvements (Retirements) Since Acquisition 1,636,000      
Carrying Amount at Which Carried [Abstract]        
Land 1,180,000      
Buildings and Improvements 8,926,000      
Total 10,106,000.00      
Accumulated Depreciation at December 31, 2018 $ 5,012,000      
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 1,781,000      
Buildings and Improvements 7,124,000      
Net Improvements (Retirements) Since Acquisition 6,831,000      
Carrying Amount at Which Carried [Abstract]        
Land 2,045,000      
Buildings and Improvements 13,691,000      
Total 15,736,000.00      
Accumulated Depreciation at December 31, 2018 $ 5,864,000      
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 1,695,000      
Buildings and Improvements 6,779,000      
Net Improvements (Retirements) Since Acquisition 1,662,000      
Carrying Amount at Which Carried [Abstract]        
Land 1,695,000      
Buildings and Improvements 8,441,000      
Total 10,136,000.00      
Accumulated Depreciation at December 31, 2018 $ 3,714,000      
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 6,927,000      
Buildings and Improvements 14,722,000      
Net Improvements (Retirements) Since Acquisition 2,448,000      
Carrying Amount at Which Carried [Abstract]        
Land 6,237,000      
Buildings and Improvements 17,860,000      
Total 24,097,000.00      
Accumulated Depreciation at December 31, 2018 $ 2,920,000      
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 4,241,000      
Buildings and Improvements 16,579,000      
Net Improvements (Retirements) Since Acquisition 5,418,000      
Carrying Amount at Which Carried [Abstract]        
Land 4,241,000      
Buildings and Improvements 21,997,000      
Total 26,238,000.00      
Accumulated Depreciation at December 31, 2018 $ 10,712,000      
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 4,222,000      
Buildings and Improvements 16,891,000      
Net Improvements (Retirements) Since Acquisition 4,827,000      
Carrying Amount at Which Carried [Abstract]        
Land 4,222,000      
Buildings and Improvements 21,718,000      
Total 25,940,000.00      
Accumulated Depreciation at December 31, 2018 $ 11,254,000      
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 2,770,000      
Buildings and Improvements 11,014,000      
Net Improvements (Retirements) Since Acquisition 2,147,000      
Carrying Amount at Which Carried [Abstract]        
Land 2,770,000      
Buildings and Improvements 13,161,000      
Total 15,931,000.00      
Accumulated Depreciation at December 31, 2018 $ 6,925,000      
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 2,772,000      
Buildings and Improvements 10,936,000      
Net Improvements (Retirements) Since Acquisition 3,660,000      
Carrying Amount at Which Carried [Abstract]        
Land 2,772,000      
Buildings and Improvements 14,596,000      
Total 17,368,000.00      
Accumulated Depreciation at December 31, 2018 $ 7,255,000      
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] | 630 Freedom Business Center Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,773,000      
Buildings and Improvements 11,144,000      
Net Improvements (Retirements) Since Acquisition 3,592,000      
Carrying Amount at Which Carried [Abstract]        
Land 2,773,000      
Buildings and Improvements 14,736,000      
Total 17,509,000.00      
Accumulated Depreciation at December 31, 2018 $ 7,517,000      
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] | 1060 First Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,712,000      
Buildings and Improvements 10,953,000      
Net Improvements (Retirements) Since Acquisition 3,539,000      
Carrying Amount at Which Carried [Abstract]        
Land 2,712,000      
Buildings and Improvements 14,492,000      
Total 17,204,000.00      
Accumulated Depreciation at December 31, 2018 $ 7,111,000      
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] | 1020 First Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,168,000      
Buildings and Improvements 8,576,000      
Net Improvements (Retirements) Since Acquisition 3,939,000      
Carrying Amount at Which Carried [Abstract]        
Land 2,168,000      
Buildings and Improvements 12,515,000      
Total 14,683,000.00      
Accumulated Depreciation at December 31, 2018 $ 6,004,000      
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] | 610 Freedom Business Center Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,017,000      
Buildings and Improvements 8,070,000      
Net Improvements (Retirements) Since Acquisition 2,547,000      
Carrying Amount at Which Carried [Abstract]        
Land 2,017,000      
Buildings and Improvements 10,617,000      
Total 12,634,000.00      
Accumulated Depreciation at December 31, 2018 $ 5,319,000      
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] | 1040 First Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,860,000      
Buildings and Improvements 11,282,000      
Net Improvements (Retirements) Since Acquisition 5,037,000      
Carrying Amount at Which Carried [Abstract]        
Land 2,860,000      
Buildings and Improvements 16,319,000      
Total 19,179,000.00      
Accumulated Depreciation at December 31, 2018 $ 7,941,000      
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 1,916,000      
Buildings and Improvements 4,378,000      
Net Improvements (Retirements) Since Acquisition 1,357,000      
Carrying Amount at Which Carried [Abstract]        
Land 1,916,000      
Buildings and Improvements 5,735,000      
Total 7,651,000.00      
Accumulated Depreciation at December 31, 2018 $ 3,418,000      
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 1,012,000      
Buildings and Improvements 4,048,000      
Net Improvements (Retirements) Since Acquisition 385,000      
Carrying Amount at Which Carried [Abstract]        
Land 1,012,000      
Buildings and Improvements 4,433,000      
Total 5,445,000.00      
Accumulated Depreciation at December 31, 2018 $ 2,527,000      
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 3,127,000      
Buildings and Improvements 20,794,000      
Net Improvements (Retirements) Since Acquisition (1,125,000)      
Carrying Amount at Which Carried [Abstract]        
Land 3,127,000      
Buildings and Improvements 19,669,000      
Total 22,796,000.00      
Accumulated Depreciation at December 31, 2018 $ 1,345,000      
Real Estate And Accumulated Depreciation Construction Year 2017      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 500 North Gulph Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,303,000      
Buildings and Improvements 5,201,000      
Net Improvements (Retirements) Since Acquisition 18,777,000      
Carrying Amount at Which Carried [Abstract]        
Land 1,303,000      
Buildings and Improvements 23,978,000      
Total 25,281,000.00      
Accumulated Depreciation at December 31, 2018 $ 82,000      
Real Estate And Accumulated Depreciation Construction Year 1979      
Real Estate And Accumulated Depreciation Year Of Acquisition 1996      
PENNSYLVANIA SUBURBS [Member] | 401 Plymouth Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 6,199,000      
Buildings and Improvements 16,131,000      
Net Improvements (Retirements) Since Acquisition 16,184,000      
Carrying Amount at Which Carried [Abstract]        
Land 6,199,000      
Buildings and Improvements 32,315,000      
Total 38,514,000.00      
Accumulated Depreciation at December 31, 2018 $ 14,477,000      
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 4,373,000      
Buildings and Improvements 24,546,000      
Net Improvements (Retirements) Since Acquisition 452,000      
Carrying Amount at Which Carried [Abstract]        
Land 4,373,000      
Buildings and Improvements 24,998,000      
Total 29,371,000.00      
Accumulated Depreciation at December 31, 2018 $ 6,941,000      
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 3,651,000      
Buildings and Improvements 14,514,000      
Net Improvements (Retirements) Since Acquisition 3,313,000      
Carrying Amount at Which Carried [Abstract]        
Land 3,651,000      
Buildings and Improvements 17,827,000      
Total 21,478,000.00      
Accumulated Depreciation at December 31, 2018 $ 7,996,000      
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 3,652,000      
Buildings and Improvements 15,288,000      
Net Improvements (Retirements) Since Acquisition 2,377,000      
Carrying Amount at Which Carried [Abstract]        
Land 3,652,000      
Buildings and Improvements 17,665,000      
Total 21,317,000.00      
Accumulated Depreciation at December 31, 2018 $ 7,422,000      
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 3,558,000      
Buildings and Improvements 14,743,000      
Net Improvements (Retirements) Since Acquisition 2,522,000      
Carrying Amount at Which Carried [Abstract]        
Land 3,558,000      
Buildings and Improvements 17,265,000      
Total 20,823,000.00      
Accumulated Depreciation at December 31, 2018 $ 7,157,000      
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 3,572,000      
Buildings and Improvements 14,435,000      
Net Improvements (Retirements) Since Acquisition 1,561,000      
Carrying Amount at Which Carried [Abstract]        
Land 3,572,000      
Buildings and Improvements 15,996,000      
Total 19,568,000.00      
Accumulated Depreciation at December 31, 2018 $ 6,517,000      
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 3,694,000      
Buildings and Improvements 5,487,000      
Net Improvements (Retirements) Since Acquisition 20,715,000      
Carrying Amount at Which Carried [Abstract]        
Land 5,405,000      
Buildings and Improvements 24,491,000      
Total 29,896,000.00      
Accumulated Depreciation at December 31, 2018 $ 5,636,000      
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 1,043,000      
Buildings and Improvements 555,000      
Net Improvements (Retirements) Since Acquisition 0      
Carrying Amount at Which Carried [Abstract]        
Land 1,043,000      
Buildings and Improvements 555,000      
Total 1,598,000.00      
Accumulated Depreciation at December 31, 2018 $ 191,000      
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 11,925,000      
Buildings and Improvements 36,986,000      
Net Improvements (Retirements) Since Acquisition 9,273,000      
Carrying Amount at Which Carried [Abstract]        
Land 11,897,000      
Buildings and Improvements 46,287,000      
Total 58,184,000.00      
Accumulated Depreciation at December 31, 2018 $ 20,332,000      
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 7,323,000      
Buildings and Improvements 28,613,000      
Net Improvements (Retirements) Since Acquisition 23,155,000      
Carrying Amount at Which Carried [Abstract]        
Land 7,323,000      
Buildings and Improvements 51,768,000      
Total 59,091,000.00      
Accumulated Depreciation at December 31, 2018 $ 28,913,000      
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 8,956,000      
Buildings and Improvements 29,811,000      
Net Improvements (Retirements) Since Acquisition 3,064,000      
Carrying Amount at Which Carried [Abstract]        
Land 8,949,000      
Buildings and Improvements 32,882,000      
Total 41,831,000.00      
Accumulated Depreciation at December 31, 2018 $ 18,605,000      
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 8,014,000      
Buildings and Improvements 16,508,000      
Net Improvements (Retirements) Since Acquisition 19,201,000      
Carrying Amount at Which Carried [Abstract]        
Land 8,609,000      
Buildings and Improvements 35,114,000      
Total 43,723,000.00      
Accumulated Depreciation at December 31, 2018 $ 16,532,000      
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 5,406,000      
Buildings and Improvements 21,390,000      
Net Improvements (Retirements) Since Acquisition 12,918,000      
Carrying Amount at Which Carried [Abstract]        
Land 5,705,000      
Buildings and Improvements 34,009,000      
Total 39,714,000.00      
Accumulated Depreciation at December 31, 2018 $ 14,908,000      
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 6,506,000      
Buildings and Improvements 25,525,000      
Net Improvements (Retirements) Since Acquisition 5,887,000      
Carrying Amount at Which Carried [Abstract]        
Land 6,578,000      
Buildings and Improvements 31,340,000      
Total 37,918,000.00      
Accumulated Depreciation at December 31, 2018 $ 11,579,000      
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 4,773,000      
Buildings and Improvements 17,961,000      
Net Improvements (Retirements) Since Acquisition 1,372,000      
Carrying Amount at Which Carried [Abstract]        
Land 4,791,000      
Buildings and Improvements 19,315,000      
Total 24,106,000.00      
Accumulated Depreciation at December 31, 2018 $ 9,731,000      
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 3,937,000      
Buildings and Improvements 15,484,000      
Net Improvements (Retirements) Since Acquisition 3,402,000      
Carrying Amount at Which Carried [Abstract]        
Land 3,942,000      
Buildings and Improvements 18,881,000      
Total 22,823,000.00      
Accumulated Depreciation at December 31, 2018 $ 9,359,000      
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 2,573,000      
Buildings and Improvements 8,338,000      
Net Improvements (Retirements) Since Acquisition 3,188,000      
Carrying Amount at Which Carried [Abstract]        
Land 2,567,000      
Buildings and Improvements 11,532,000      
Total 14,099,000.00      
Accumulated Depreciation at December 31, 2018 $ 6,206,000      
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] | Four Tower Bridge (200 Barr Harbor Drive)        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 9,526,000      
Initial Cost [Abstract]        
Land 6,000,000      
Buildings and Improvements 14,734,000      
Net Improvements (Retirements) Since Acquisition 19,000      
Carrying Amount at Which Carried [Abstract]        
Land 6,000,000      
Buildings and Improvements 14,753,000      
Total 20,753,000.00      
Accumulated Depreciation at December 31, 2018 $ 369,000      
Real Estate And Accumulated Depreciation Construction Year 1998      
Real Estate And Accumulated Depreciation Year Of Acquisition 2018      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 170 Radnor Chester Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,514,000      
Buildings and Improvements 8,147,000      
Net Improvements (Retirements) Since Acquisition 2,165,000      
Carrying Amount at Which Carried [Abstract]        
Land 2,509,000      
Buildings and Improvements 10,317,000      
Total 12,826,000.00      
Accumulated Depreciation at December 31, 2018 $ 4,210,000      
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 3,366,000      
Buildings and Improvements 0      
Net Improvements (Retirements) Since Acquisition 3,572,000      
Carrying Amount at Which Carried [Abstract]        
Land 3,366,000      
Buildings and Improvements 3,572,000      
Total 6,938,000.00      
Accumulated Depreciation at December 31, 2018 $ 636,000      
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 6,251,000      
Buildings and Improvements 25,209,000      
Net Improvements (Retirements) Since Acquisition 3,633,000      
Carrying Amount at Which Carried [Abstract]        
Land 6,251,000      
Buildings and Improvements 28,842,000      
Total 35,093,000.00      
Accumulated Depreciation at December 31, 2018 $ 9,723,000      
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 3,557,000      
Buildings and Improvements 14,249,000      
Net Improvements (Retirements) Since Acquisition 3,372,000      
Carrying Amount at Which Carried [Abstract]        
Land 3,557,000      
Buildings and Improvements 17,621,000      
Total 21,178,000.00      
Accumulated Depreciation at December 31, 2018 $ 5,673,000      
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 208,570,000      
Net Improvements (Retirements) Since Acquisition (13,907,000)      
Carrying Amount at Which Carried [Abstract]        
Land 12,586,000      
Buildings and Improvements 182,077,000      
Total 194,663,000.00      
Accumulated Depreciation at December 31, 2018 $ 65,851,000      
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 98,188,000      
Net Improvements (Retirements) Since Acquisition 63,532,000      
Carrying Amount at Which Carried [Abstract]        
Land 25,195,000      
Buildings and Improvements 136,525,000      
Total 161,720,000.00      
Accumulated Depreciation at December 31, 2018 $ 34,908,000      
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 $ 110,518,000      
Initial Cost [Abstract]        
Land 15,323,000      
Buildings and Improvements 120,200,000      
Net Improvements (Retirements) Since Acquisition 24,144,000      
Carrying Amount at Which Carried [Abstract]        
Land 15,323,000      
Buildings and Improvements 144,344,000      
Total 159,667,000.00      
Accumulated Depreciation at December 31, 2018 $ 22,184,000      
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 14,496,000      
Buildings and Improvements 107,736,000      
Net Improvements (Retirements) Since Acquisition 28,657,000      
Carrying Amount at Which Carried [Abstract]        
Land 14,473,000      
Buildings and Improvements 136,416,000      
Total 150,889,000.00      
Accumulated Depreciation at December 31, 2018 $ 56,850,000      
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 $ 82,805,000      
Initial Cost [Abstract]        
Land 16,066,000      
Buildings and Improvements 100,255,000      
Net Improvements (Retirements) Since Acquisition 19,053,000      
Carrying Amount at Which Carried [Abstract]        
Land 16,066,000      
Buildings and Improvements 119,308,000      
Total 135,374,000.00      
Accumulated Depreciation at December 31, 2018 $ 45,211,000      
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 $ 120,183,000      
Initial Cost [Abstract]        
Land 15,161,000      
Buildings and Improvements 105,021,000      
Net Improvements (Retirements) Since Acquisition 27,997,000      
Carrying Amount at Which Carried [Abstract]        
Land 15,160,000      
Buildings and Improvements 133,019,000      
Total 148,179,000.00      
Accumulated Depreciation at December 31, 2018 $ 19,848,000      
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 (129 South 30th Street)        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 0      
Buildings and Improvements 76,008,000      
Net Improvements (Retirements) Since Acquisition 14,183,000      
Carrying Amount at Which Carried [Abstract]        
Land 0      
Buildings and Improvements 90,191,000      
Total 90,191,000.00      
Accumulated Depreciation at December 31, 2018 $ 17,015,000      
Real Estate And Accumulated Depreciation Construction Year 2010      
Depreciable Life (Years) 40 years      
Philadelphia CBD [Member] | 1900 Market Street [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 7,768,000      
Buildings and Improvements 17,263,000      
Net Improvements (Retirements) Since Acquisition 51,364,000      
Carrying Amount at Which Carried [Abstract]        
Land 7,768,000      
Buildings and Improvements 68,627,000      
Total 76,395,000.00      
Accumulated Depreciation at December 31, 2018 $ 11,327,000      
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 21,417,000      
Net Improvements (Retirements) Since Acquisition 8,100,000      
Carrying Amount at Which Carried [Abstract]        
Land 0      
Buildings and Improvements 29,517,000      
Total 29,517,000.00      
Accumulated Depreciation at December 31, 2018 $ 9,013,000      
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 14,401,000      
Net Improvements (Retirements) Since Acquisition (14,401,000)      
Carrying Amount at Which Carried [Abstract]        
Land 0      
Buildings and Improvements 0      
Total 0      
Accumulated Depreciation at December 31, 2018 $ 0      
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 13,365,000      
Buildings and Improvements 5,791,000      
Net Improvements (Retirements) Since Acquisition 1,952,000      
Carrying Amount at Which Carried [Abstract]        
Land 13,365,000      
Buildings and Improvements 7,743,000      
Total 21,108,000.00      
Accumulated Depreciation at December 31, 2018 $ 4,601,000      
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 400,294,000      
Net Improvements (Retirements) Since Acquisition 12,964,000      
Carrying Amount at Which Carried [Abstract]        
Land 0      
Buildings and Improvements 413,258,000      
Total 413,258,000.00      
Accumulated Depreciation at December 31, 2018 $ 31,439,000      
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 18,827,000      
Buildings and Improvements 0      
Net Improvements (Retirements) Since Acquisition 6,594,000      
Carrying Amount at Which Carried [Abstract]        
Land 18,854,000      
Buildings and Improvements 6,567,000      
Total 25,421,000.00      
Accumulated Depreciation at December 31, 2018 $ 547,000      
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 18,924,000      
Buildings and Improvements 13,080,000      
Net Improvements (Retirements) Since Acquisition 600,000      
Carrying Amount at Which Carried [Abstract]        
Land 18,924,000      
Buildings and Improvements 13,680,000      
Total 32,604,000.00      
Accumulated Depreciation at December 31, 2018 $ 1,720,000      
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] | The Bulletin Building (3025 Market Street) [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 0      
Buildings and Improvements 24,377,000      
Net Improvements (Retirements) Since Acquisition 6,968,000      
Carrying Amount at Which Carried [Abstract]        
Land 0      
Buildings and Improvements 31,345,000      
Total 31,345,000.00      
Accumulated Depreciation at December 31, 2018 $ 978,000      
Real Estate And Accumulated Depreciation Construction Year 1953      
Real Estate And Accumulated Depreciation Year Of Acquisition 2017      
Depreciable Life (Years) 40 years      
Philadelphia CBD [Member] | 3001-3003 JFK Boulevard [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 0      
Buildings and Improvements 0      
Net Improvements (Retirements) Since Acquisition 78,000      
Carrying Amount at Which Carried [Abstract]        
Land 0      
Buildings and Improvements 78,000      
Total 78,000.00      
Accumulated Depreciation at December 31, 2018 $ 2,000      
Real Estate And Accumulated Depreciation Year Of Acquisition 2018      
Philadelphia CBD [Member] | 3025 JFK Boulevard [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 0      
Buildings and Improvements 0      
Net Improvements (Retirements) Since Acquisition 0      
Carrying Amount at Which Carried [Abstract]        
Land 0      
Buildings and Improvements 0      
Total 0      
Accumulated Depreciation at December 31, 2018 $ 0      
Real Estate And Accumulated Depreciation Year Of Acquisition 2018      
Metropolitan DC [Member] | 6600 Rockledge Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 0      
Buildings and Improvements 37,421,000      
Net Improvements (Retirements) Since Acquisition 9,349,000      
Carrying Amount at Which Carried [Abstract]        
Land 0      
Buildings and Improvements 46,770,000      
Total 46,770,000.00      
Accumulated Depreciation at December 31, 2018 $ 13,469,000      
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 0      
Buildings and Improvements 0      
Net Improvements (Retirements) Since Acquisition 82,248,000      
Carrying Amount at Which Carried [Abstract]        
Land 16,129,000      
Buildings and Improvements 66,119,000      
Total 82,248,000.00      
Accumulated Depreciation at December 31, 2018 $ 20,429,000      
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] | 1676 International Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 18,437,000      
Buildings and Improvements 97,538,000      
Net Improvements (Retirements) Since Acquisition 3,412,000      
Carrying Amount at Which Carried [Abstract]        
Land 18,785,000      
Buildings and Improvements 100,602,000      
Total 119,387,000.00      
Accumulated Depreciation at December 31, 2018 $ 25,864,000      
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 7,952,000      
Buildings and Improvements 33,964,000      
Net Improvements (Retirements) Since Acquisition 4,559,000      
Carrying Amount at Which Carried [Abstract]        
Land 8,102,000      
Buildings and Improvements 38,373,000      
Total 46,475,000.00      
Accumulated Depreciation at December 31, 2018 $ 10,083,000      
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 5,167,000      
Buildings and Improvements 31,110,000      
Net Improvements (Retirements) Since Acquisition 4,538,000      
Carrying Amount at Which Carried [Abstract]        
Land 5,237,000      
Buildings and Improvements 35,578,000      
Total 40,815,000.00      
Accumulated Depreciation at December 31, 2018 $ 10,624,000      
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 5,059,000      
Buildings and Improvements 29,668,000      
Net Improvements (Retirements) Since Acquisition 8,034,000      
Carrying Amount at Which Carried [Abstract]        
Land 5,154,000      
Buildings and Improvements 37,607,000      
Total 42,761,000.00      
Accumulated Depreciation at December 31, 2018 $ 12,388,000      
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 4,649,000      
Buildings and Improvements 26,952,000      
Net Improvements (Retirements) Since Acquisition 18,854,000      
Carrying Amount at Which Carried [Abstract]        
Land 4,733,000      
Buildings and Improvements 45,722,000      
Total 50,455,000.00      
Accumulated Depreciation at December 31, 2018 $ 14,723,000      
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 7,797,000      
Buildings and Improvements 47,817,000      
Net Improvements (Retirements) Since Acquisition (2,381,000)      
Carrying Amount at Which Carried [Abstract]        
Land 5,566,000      
Buildings and Improvements 47,667,000      
Total 53,233,000.00      
Accumulated Depreciation at December 31, 2018 $ 19,418,000      
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 4,316,000      
Buildings and Improvements 30,885,000      
Net Improvements (Retirements) Since Acquisition 6,826,000      
Carrying Amount at Which Carried [Abstract]        
Land 4,397,000      
Buildings and Improvements 37,630,000      
Total 42,027,000.00      
Accumulated Depreciation at December 31, 2018 $ 11,046,000      
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 3,755,000      
Buildings and Improvements 22,702,000      
Net Improvements (Retirements) Since Acquisition 117,000      
Carrying Amount at Which Carried [Abstract]        
Land 3,755,000      
Buildings and Improvements 22,819,000      
Total 26,574,000.00      
Accumulated Depreciation at December 31, 2018 $ 2,489,000      
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 2,732,000      
Buildings and Improvements 16,305,000      
Net Improvements (Retirements) Since Acquisition 1,562,000      
Carrying Amount at Which Carried [Abstract]        
Land 2,732,000      
Buildings and Improvements 17,867,000      
Total 20,599,000.00      
Accumulated Depreciation at December 31, 2018 $ 2,268,000      
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 3,688,000      
Buildings and Improvements 22,348,000      
Net Improvements (Retirements) Since Acquisition 115,000      
Carrying Amount at Which Carried [Abstract]        
Land 3,688,000      
Buildings and Improvements 22,463,000      
Total 26,151,000.00      
Accumulated Depreciation at December 31, 2018 $ 2,449,000      
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 2,614,000      
Buildings and Improvements 15,740,000      
Net Improvements (Retirements) Since Acquisition 81,000      
Carrying Amount at Which Carried [Abstract]        
Land 2,614,000      
Buildings and Improvements 15,821,000      
Total 18,435,000.00      
Accumulated Depreciation at December 31, 2018 $ 1,725,000      
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 3,689,000      
Buildings and Improvements 22,354,000      
Net Improvements (Retirements) Since Acquisition 115,000      
Carrying Amount at Which Carried [Abstract]        
Land 3,689,000      
Buildings and Improvements 22,469,000      
Total 26,158,000.00      
Accumulated Depreciation at December 31, 2018 $ 2,450,000      
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 2,676,000      
Buildings and Improvements 15,972,000      
Net Improvements (Retirements) Since Acquisition 13,396,000      
Carrying Amount at Which Carried [Abstract]        
Land 2,676,000      
Buildings and Improvements 29,368,000      
Total 32,044,000.00      
Accumulated Depreciation at December 31, 2018 $ 1,387,000      
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 1,400,000      
Buildings and Improvements 7,422,000      
Net Improvements (Retirements) Since Acquisition 100,000      
Carrying Amount at Which Carried [Abstract]        
Land 1,400,000      
Buildings and Improvements 7,522,000      
Total 8,922,000.00      
Accumulated Depreciation at December 31, 2018 $ 828,000      
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 19,826,000      
Net Improvements (Retirements) Since Acquisition 102,000      
Carrying Amount at Which Carried [Abstract]        
Land 0      
Buildings and Improvements 19,928,000      
Total 19,928,000.00      
Accumulated Depreciation at December 31, 2018 $ 2,884,000      
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] | One Barton Skyway (1501 South MoPac Expressway) [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 10,496,000      
Buildings and Improvements 47,670,000      
Net Improvements (Retirements) Since Acquisition (39,000)      
Carrying Amount at Which Carried [Abstract]        
Land 10,495,000      
Buildings and Improvements 47,632,000      
Total 58,127,000.00      
Accumulated Depreciation at December 31, 2018 $ 103,000      
Real Estate And Accumulated Depreciation Construction Year 1999      
Real Estate And Accumulated Depreciation Year Of Acquisition 2018      
Depreciable Life (Years) 40 years      
Austin, Texas [Member] | Two Barton Skyway (1601 South MoPac Expressway) [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 10,849,000      
Buildings and Improvements 53,868,000      
Net Improvements (Retirements) Since Acquisition (40,000)      
Carrying Amount at Which Carried [Abstract]        
Land 10,848,000      
Buildings and Improvements 53,829,000      
Total 64,677,000.00      
Accumulated Depreciation at December 31, 2018 $ 116,000      
Real Estate And Accumulated Depreciation Construction Year 2000      
Real Estate And Accumulated Depreciation Year Of Acquisition 2018      
Depreciable Life (Years) 40 years      
Austin, Texas [Member] | Three Barton Skyway (1221 South MoPac Expressway) [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 10,374,000      
Buildings and Improvements 47,624,000      
Net Improvements (Retirements) Since Acquisition (39,000)      
Carrying Amount at Which Carried [Abstract]        
Land 10,373,000      
Buildings and Improvements 47,586,000      
Total 57,959,000.00      
Accumulated Depreciation at December 31, 2018 $ 100,000      
Real Estate And Accumulated Depreciation Construction Year 2001      
Real Estate And Accumulated Depreciation Year Of Acquisition 2018      
Depreciable Life (Years) 40 years      
Austin, Texas [Member] | Four Barton Skyway (1301 South MoPac Expressway) [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 13,301,000      
Buildings and Improvements 57,041,000      
Net Improvements (Retirements) Since Acquisition (40,000)      
Carrying Amount at Which Carried [Abstract]        
Land 13,300,000      
Buildings and Improvements 57,002,000      
Total 70,302,000.00      
Accumulated Depreciation at December 31, 2018 $ 121,000      
Real Estate And Accumulated Depreciation Construction Year 2001      
Real Estate And Accumulated Depreciation Year Of Acquisition 2018      
Depreciable Life (Years) 40 years      
Austin, Texas [Member] | Four Points Centre (11305 Four Points Drive) [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 7,800,000      
Buildings and Improvements 43,581,000      
Net Improvements (Retirements) Since Acquisition (40,000)      
Carrying Amount at Which Carried [Abstract]        
Land 7,800,000      
Buildings and Improvements 43,541,000      
Total 51,341,000.00      
Accumulated Depreciation at December 31, 2018 $ 91,000      
Real Estate And Accumulated Depreciation Construction Year 2008      
Real Estate And Accumulated Depreciation Year Of Acquisition 2018      
Depreciable Life (Years) 40 years      
Austin, Texas [Member] | River Place - Building 1 (6500 River Place Boulevard) [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,004,000      
Buildings and Improvements 17,680,000      
Net Improvements (Retirements) Since Acquisition (39,000)      
Carrying Amount at Which Carried [Abstract]        
Land 2,004,000      
Buildings and Improvements 17,641,000      
Total 19,645,000.00      
Accumulated Depreciation at December 31, 2018 $ 36,000      
Real Estate And Accumulated Depreciation Construction Year 2000      
Real Estate And Accumulated Depreciation Year Of Acquisition 2018      
Depreciable Life (Years) 40 years      
Austin, Texas [Member] | River Place - Building 2 (6500 River Place Boulevard) [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 3,137,000      
Buildings and Improvements 29,254,000      
Net Improvements (Retirements) Since Acquisition (40,000)      
Carrying Amount at Which Carried [Abstract]        
Land 3,137,000      
Buildings and Improvements 29,214,000      
Total 32,351,000.00      
Accumulated Depreciation at December 31, 2018 $ 61,000      
Real Estate And Accumulated Depreciation Construction Year 2000      
Real Estate And Accumulated Depreciation Year Of Acquisition 2018      
Depreciable Life (Years) 40 years      
Austin, Texas [Member] | River Place - Building 3 (6500 River Place Boulevard) [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 3,064,000      
Buildings and Improvements 26,705,000      
Net Improvements (Retirements) Since Acquisition (40,000)      
Carrying Amount at Which Carried [Abstract]        
Land 3,064,000      
Buildings and Improvements 26,665,000      
Total 29,729,000.00      
Accumulated Depreciation at December 31, 2018 $ 56,000      
Real Estate And Accumulated Depreciation Construction Year 2000      
Real Estate And Accumulated Depreciation Year Of Acquisition 2018      
Depreciable Life (Years) 40 years      
Austin, Texas [Member] | River Place - Building 4 (6500 River Place Boulevard [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,273,000      
Buildings and Improvements 18,617,000      
Net Improvements (Retirements) Since Acquisition (39,000)      
Carrying Amount at Which Carried [Abstract]        
Land 2,273,000      
Buildings and Improvements 18,578,000      
Total 20,851,000.00      
Accumulated Depreciation at December 31, 2018 $ 39,000      
Real Estate And Accumulated Depreciation Construction Year 2000      
Real Estate And Accumulated Depreciation Year Of Acquisition 2018      
Depreciable Life (Years) 40 years      
Austin, Texas [Member] | River Place - Building 5 (6500 River Place Boulevard) [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,752,000      
Buildings and Improvements 14,315,000      
Net Improvements (Retirements) Since Acquisition (39,000)      
Carrying Amount at Which Carried [Abstract]        
Land 1,752,000      
Buildings and Improvements 14,276,000      
Total 16,028,000.00      
Accumulated Depreciation at December 31, 2018 $ 30,000      
Real Estate And Accumulated Depreciation Construction Year 2001      
Real Estate And Accumulated Depreciation Year Of Acquisition 2018      
Depreciable Life (Years) 40 years      
Austin, Texas [Member] | River Place - Building 6 (6500 River Place Boulevard) [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,598,000      
Buildings and Improvements 12,945,000      
Net Improvements (Retirements) Since Acquisition (39,000)      
Carrying Amount at Which Carried [Abstract]        
Land 1,598,000      
Buildings and Improvements 12,906,000      
Total 14,504,000.00      
Accumulated Depreciation at December 31, 2018 $ 27,000      
Real Estate And Accumulated Depreciation Construction Year 2001      
Real Estate And Accumulated Depreciation Year Of Acquisition 2018      
Depreciable Life (Years) 40 years      
Austin, Texas [Member] | River Place - Building 7 (6500 River Place Boulevard) [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,801,000      
Buildings and Improvements 16,486,000      
Net Improvements (Retirements) Since Acquisition (39,000)      
Carrying Amount at Which Carried [Abstract]        
Land 1,801,000      
Buildings and Improvements 16,447,000      
Total 18,248,000.00      
Accumulated Depreciation at December 31, 2018 $ 35,000      
Real Estate And Accumulated Depreciation Construction Year 2002      
Real Estate And Accumulated Depreciation Year Of Acquisition 2018      
Depreciable Life (Years) 40 years      
Austin, Texas [Member] | Quarry Lake II (4516 Seton Center Parkway) [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 3,970,000      
Buildings and Improvements 30,546,000      
Net Improvements (Retirements) Since Acquisition 604,000      
Carrying Amount at Which Carried [Abstract]        
Land 3,867,000      
Buildings and Improvements 31,253,000      
Total 35,120,000.00      
Accumulated Depreciation at December 31, 2018 $ 0      
Real Estate And Accumulated Depreciation Construction Year 1998      
Real Estate And Accumulated Depreciation Year Of Acquisition 2018      
Depreciable Life (Years) 40 years      
Other [Member] | 10 Foster Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 244,000      
Buildings and Improvements 971,000      
Net Improvements (Retirements) Since Acquisition 69,000      
Carrying Amount at Which Carried [Abstract]        
Land 244,000      
Buildings and Improvements 1,040,000      
Total 1,284,000.00      
Accumulated Depreciation at December 31, 2018 $ 597,000      
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 231,000      
Buildings and Improvements 921,000      
Net Improvements (Retirements) Since Acquisition 31,000      
Carrying Amount at Which Carried [Abstract]        
Land 231,000      
Buildings and Improvements 952,000      
Total 1,183,000.00      
Accumulated Depreciation at December 31, 2018 $ 522,000      
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 185,000      
Buildings and Improvements 730,000      
Net Improvements (Retirements) Since Acquisition 11,000      
Carrying Amount at Which Carried [Abstract]        
Land 185,000      
Buildings and Improvements 741,000      
Total 926,000.00      
Accumulated Depreciation at December 31, 2018 $ 741,000      
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 183,000      
Buildings and Improvements 726,000      
Net Improvements (Retirements) Since Acquisition 6,000      
Carrying Amount at Which Carried [Abstract]        
Land 183,000      
Buildings and Improvements 732,000      
Total 915,000.00      
Accumulated Depreciation at December 31, 2018 $ 732,000      
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 93,000      
Buildings and Improvements 364,000      
Net Improvements (Retirements) Since Acquisition 8,000      
Carrying Amount at Which Carried [Abstract]        
Land 93,000      
Buildings and Improvements 372,000      
Total 465,000.00      
Accumulated Depreciation at December 31, 2018 $ 372,000      
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 21,000      
Buildings and Improvements 81,000      
Net Improvements (Retirements) Since Acquisition 2,000      
Carrying Amount at Which Carried [Abstract]        
Land 21,000      
Buildings and Improvements 83,000      
Total 104,000.00      
Accumulated Depreciation at December 31, 2018 $ 59,000      
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 9,000      
Buildings and Improvements 32,000      
Net Improvements (Retirements) Since Acquisition 3,000      
Carrying Amount at Which Carried [Abstract]        
Land 9,000      
Buildings and Improvements 35,000      
Total 44,000.00      
Accumulated Depreciation at December 31, 2018 $ 25,000      
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 2,732,000      
Buildings and Improvements 10,942,000      
Net Improvements (Retirements) Since Acquisition 376,000      
Carrying Amount at Which Carried [Abstract]        
Land 2,732,000      
Buildings and Improvements 11,318,000      
Total 14,050,000.00      
Accumulated Depreciation at December 31, 2018 $ 11,064,000      
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 696,000      
Buildings and Improvements 2,802,000      
Net Improvements (Retirements) Since Acquisition 3,701,000      
Carrying Amount at Which Carried [Abstract]        
Land 705,000      
Buildings and Improvements 6,494,000      
Total 7,199,000.00      
Accumulated Depreciation at December 31, 2018 $ 3,124,000      
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 532,000      
Buildings and Improvements 2,052,000      
Net Improvements (Retirements) Since Acquisition 405,000      
Carrying Amount at Which Carried [Abstract]        
Land 532,000      
Buildings and Improvements 2,457,000      
Total 2,989,000.00      
Accumulated Depreciation at December 31, 2018 $ 1,445,000      
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 6,141,000      
Buildings and Improvements 21,140,000      
Net Improvements (Retirements) Since Acquisition 7,969,000      
Carrying Amount at Which Carried [Abstract]        
Land 6,141,000      
Buildings and Improvements 29,109,000      
Total 35,250,000.00      
Accumulated Depreciation at December 31, 2018 $ 11,593,000      
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 6,369,000      
Buildings and Improvements 13,739,000      
Net Improvements (Retirements) Since Acquisition 1,991,000      
Carrying Amount at Which Carried [Abstract]        
Land 6,369,000      
Buildings and Improvements 15,729,000      
Total 22,099,000.00      
Accumulated Depreciation at December 31, 2018 $ 8,202,000      
Real Estate And Accumulated Depreciation Construction Year 1989      
Real Estate And Accumulated Depreciation Year Of Acquisition 2004      
Depreciable Life (Years) 23 years