Investment in Unconsolidated Entities - Schedule Investment in Unconsolidated Entities (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Assets
Other assets	$ 21,698						$ 21,698		$ 22,836
Total assets	2,528,615						2,528,615		2,507,188
Liabilities and equity:
Other liabilities	36,847						36,847		29,039
Equity	1,749,428	$ 1,759,532	$ 1,788,123	$ 1,820,943	$ 1,810,697	$ 1,829,263	1,749,428	$ 1,820,943	1,819,601	$ 1,852,314
Total liabilities and stockholders' equity	2,528,615						2,528,615		2,507,188
Operating expenses
Total income	52,545			57,997			146,622	172,146
Depreciation and amortization	(22,170)			(24,253)			(66,697)	(71,807)
Real estate taxes	(8,940)			(9,000)			(24,647)	(27,009)
Interest expense, net	(4,594)			(6,077)			(14,327)	(17,182)
General and administrative	(10,106)			(8,379)			(25,688)	(25,788)
Loss on extinguishment of debt	0			(2,092)			(2,543)	(2,901)
Provision for asset impairment	0			(2,359)			(9,002)	(2,359)
Net income (loss)	1,677	$ (9,611)	$ (3,486)	24,926	$ (5,722)	$ (9,499)	(11,420)	9,705
Equity in earnings (losses) of unconsolidated entities	951			572			1,591	(48)
IAGM Retail Fund I, LLC
Liabilities and equity:
Accumulated amortization	19						19
Carrying value of investments in unconsolidated entities	116,706						116,706		118,861
Equity Method Investment, Nonconsolidated Investee or Group of Investees
Operating expenses
Net income (loss)	1,715			1,041			4,685	(5,033)
Company's share of net income (loss)	943			572			2,530	(4,451)
Equity in earnings (losses) of unconsolidated entities	951			572			1,591	(48)
Equity Method Investment, Nonconsolidated Investee or Group of Investees | IAGM Retail Fund I, LLC
Assets
Real estate assets, net of accumulated depreciation	400,521						400,521		425,585
Other assets	71,437						71,437		66,437
Total assets	471,958						471,958		492,022
Liabilities and equity:
Mortgages payable, net	242,252						242,252		256,732
Other liabilities	17,368						17,368		20,765
Equity	212,338						212,338		214,525
Total liabilities and stockholders' equity	471,958						471,958		492,022
Company's share of equity	117,645						117,645		118,861
Deferred gain, net of accumulated amortization of $19	(939)						(939)		0
Carrying value of investments in unconsolidated entities	116,706						116,706		$ 118,861
Operating expenses
Total income	11,911			13,284			34,449	40,037
Depreciation and amortization	(3,837)			(4,687)			(12,495)	(15,626)
Property operating	(2,234)			(2,341)			(6,057)	(6,364)
Real estate taxes	(2,267)			(2,472)			(6,875)	(7,433)
Interest expense, net	(1,744)			(2,630)			(5,712)	(8,418)
General and administrative	(114)			(113)			(358)	(358)
Gain (loss) on sale of real estate, net	0			0			1,741	(559)
Loss on extinguishment of debt	0			0			(8)	0
Provision for asset impairment	0			0			0	(1,443)
Net income (loss)	1,715			1,041			4,685	(164)
Outside basis adjustment for investee's sale of real estate	8			0			(939)	0
Equity Method Investment, Nonconsolidated Investee or Group of Investees | Downtown Railyard Venture, LLC
Operating expenses
Net income (loss)	0			0			0	(4,869)
Outside basis adjustment for investee's sale of real estate	$ 0			$ 0			$ 0	$ 4,403