Impairment and Disposal of Long-Lived Assets Discontinued Operations (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) bldg	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) bldg	Sep. 30, 2017 USD ($)	Dec. 31, 2017 bldg
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Gain on property dispositions	$ 94,878,000	$ 0	$ 184,689,000	$ 0
Net income	103,971,000	3,125,000	209,945,000	17,942,000
Income from discontinued operations available to common shareholders	$ 101,548,000	3,052,000	$ 205,053,000	17,522,000
Discontinued Operations, Held-for-sale [Member]
Discontined Operations Income Statement [Line Items]
Number of Real Estate Properties | bldg	13		13
Discontinued Operations, Disposed of by Sale [Member]
Discontined Operations Income Statement [Line Items]
Number of Real Estate Properties | bldg	28		28		2
Discontinued Operations [Member]
Discontined Operations Income Statement [Line Items]
Expected timing of disposal			one year
Capital expenditures	$ 759,000	14,000,000	$ 5,400,000	46,400,000
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Revenues	14,013,000	22,066,000	45,511,000	64,184,000
Operating expenses	(3,239,000)	(6,834,000)	(11,099,000)	(21,783,000)
Depreciation and amortization	(295,000)	(4,828,000)	(4,444,000)	(13,907,000)
Disposal Group, Including Discontinued Operation, Impairment Expense	0	(5,714,000)	0	(5,714,000)
Interest and other income	10,000	47,000	37,000	98,000
Interest expense	(1,396,000)	(1,612,000)	(4,749,000)	(4,936,000)
Net income before (loss) gain on property dispositions	9,093,000	3,125,000	25,256,000	17,942,000
Gain on property dispositions	94,878,000	0	184,689,000	0
Net income	103,971,000	3,125,000	209,945,000	17,942,000
Noncontrolling interest - operating partnership	(2,423,000)	(73,000)	(4,892,000)	(420,000)
Income from discontinued operations available to common shareholders	$ 101,548,000	$ 3,052,000	$ 205,053,000	$ 17,522,000
Florida [Member] | Discontinued Operations, Held-for-sale [Member]
Discontined Operations Income Statement [Line Items]
Number of Real Estate Properties | bldg	3		3
Southeastern PA [Member] | Discontinued Operations, Held-for-sale [Member]
Discontined Operations Income Statement [Line Items]
Number of Real Estate Properties | bldg	8		8
DC Metro [Member] | Discontinued Operations, Held-for-sale [Member]
Discontined Operations Income Statement [Line Items]
Number of Real Estate Properties | bldg	2		2