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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 |  |  |

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