Impairment and Disposal of Long-Lived Assets - Discontinued Operations (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($) a	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) a	Sep. 30, 2018 USD ($)	Sep. 30, 2019 properties	Sep. 30, 2019 bldg	Dec. 31, 2018 bldg
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Income taxes	$ (878)	$ (444)	$ (1,449)	$ (1,884)
Loss on debt extinguishment			(7,618)	0
Gain on property dispositions	37,200	94,900	87,400	184,700
Income from discontinued operations	42,189	109,198	105,905	224,531
Noncontrolling interest - consolidated joint ventures	(3)	(232)	(170)	(1,010)
Income from discontinued operations available to common shareholders	41,273	106,430	103,315	218,887
Discontinued operations
Discontined Operations Income Statement [Line Items]
Capital expenditures	100	10,800	3,600	38,300
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Revenues	8,498	23,911	33,593	73,613
Operating expenses	(3,117)	(5,329)	(10,903)	(17,276)
Depreciation and amortization	0	(2,666)	(2,129)	(11,138)
Impairment charges - real estate assets	(7,190)	0	(21,482)	0
Interest and other income (expense)	4	(31)	(57)	(132)
Income taxes	(5)	(28)	(298)	(83)
Loss on debt extinguishment	0	0	(7,618)	0
Interest expense	(429)	(1,537)	(1,653)	(5,142)
Gain on property dispositions	44,428	94,878	116,452	184,689
Income from discontinued operations	42,189	109,198	105,905	224,531
Noncontrolling interest - operating partnership	(916)	(2,544)	(2,404)	(5,232)
Noncontrolling interest - consolidated joint ventures	0	(224)	(186)	(412)
Income from discontinued operations available to common shareholders	$ 41,273	$ 106,430	$ 103,315	$ 218,887
Properties included in discontinued operations, held for sale
Discontined Operations Income Statement [Line Items]
Number of buildings (in buildings) | bldg						9
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						6
Properties included in discontinued operations, held for sale | United Kingdom
Discontined Operations Income Statement [Line Items]
Number of buildings (in buildings) | bldg						2
Properties included in discontinued operations, sold
Discontined Operations Income Statement [Line Items]
Number of buildings (in buildings) | bldg						13	37
Properties included in discontinued operations, total
Discontined Operations Income Statement [Line Items]
Number of buildings (in buildings) | bldg						59
Properties included in continuing operations, held for sale
Discontined Operations Income Statement [Line Items]
Number of buildings (in buildings) | bldg						1
Properties included in continuing operations, sold
Discontined Operations Income Statement [Line Items]
Number of buildings (in buildings) | bldg						1	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)					3	10
Acres of Developable Land (in acres) | a	74.4		74.4
Properties sold or classified as held for sale, sold
Discontined Operations Income Statement [Line Items]
Number of buildings (in buildings) | bldg						14	39
Properties sold or classified as held for sale, total
Discontined Operations Income Statement [Line Items]
Number of buildings (in buildings) | bldg						63