Impairment and Disposal of Long-Lived Assets - Discontinued Operations (Details)	3 Months Ended
	Mar. 31, 2019 USD ($) bldg	Mar. 31, 2018 USD ($)	Dec. 31, 2018 bldg
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Income taxes	$ (832,000)	$ (528,000)
Gain on property dispositions	8,700,000	90,000,000
Income from discontinued operations	14,786,000	103,007,000
Income from discontinued operations available to common shareholders	14,443,000	100,607,000
Discontinued operations
Discontined Operations Income Statement [Line Items]
Capital expenditures	1,500,000	16,200,000
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Revenues	13,264,000	25,631,000
Operating expenses	(4,288,000)	(6,572,000)
Depreciation and amortization	(1,981,000)	(5,246,000)
Impairment expense	(10,274,000)	0
Interest and other income	(158,000)	(3,000)
Income taxes	(32,000)	(26,000)
Interest expense	(686,000)	(789,000)
Gain on property dispositions	18,941,000	90,012,000
Income from discontinued operations	14,786,000	103,007,000
Noncontrolling interest - operating partnership	(343,000)	(2,400,000)
Income from discontinued operations available to common shareholders	$ 14,443,000	$ 100,607,000
Properties included in discontinued operations, held for sale
Discontined Operations Income Statement [Line Items]
Number of buildings (in buildings) | bldg	16
Properties included in discontinued operations, held for sale | Southeastern PA
Discontined Operations Income Statement [Line Items]
Number of buildings (in buildings) | bldg	5
Properties included in discontinued operations, held for sale | Chicago/Minneapolis
Discontined Operations Income Statement [Line Items]
Number of buildings (in buildings) | bldg	1
Properties included in discontinued operations, held for sale | Philadelphia
Discontined Operations Income Statement [Line Items]
Number of buildings (in buildings) | bldg	9
Properties included in discontinued operations, held for sale | DC Metro
Discontined Operations Income Statement [Line Items]
Number of buildings (in buildings) | bldg	1
Properties included in discontinued operations, sold
Discontined Operations Income Statement [Line Items]
Number of buildings (in buildings) | bldg	4		37
Properties included in discontinued operations, total
Discontined Operations Income Statement [Line Items]
Number of buildings (in buildings) | bldg	57
Properties included in continuing operations, held for sale
Discontined Operations Income Statement [Line Items]
Number of buildings (in buildings) | bldg	2
Properties included in continuing operations, sold
Discontined Operations Income Statement [Line Items]
Number of buildings (in buildings) | bldg	0		2
Properties included in continuing operations, total
Discontined Operations Income Statement [Line Items]
Number of buildings (in buildings) | bldg	4
Properties sold or classified as held for sale, held for sale
Discontined Operations Income Statement [Line Items]
Number of buildings (in buildings) | bldg	18
Properties sold or classified as held for sale, sold
Discontined Operations Income Statement [Line Items]
Number of buildings (in buildings) | bldg	4		39
Properties sold or classified as held for sale, total
Discontined Operations Income Statement [Line Items]
Number of buildings (in buildings) | bldg	61