Segment Information - Additional Information (Details) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Segment information
Number of reportable segments (segment) | segment										2
Total revenues		$ 361,342	$ 374,600	$ 398,150	$ 377,381	$ 374,242	$ 416,681	$ 617,614	$ 455,444	$ 1,511,473	$ 1,863,981	$ 1,662,829
Income from continuing operations before equity in net gain on sale of interest in unconsolidated joint venture/real estate, purchase price fair value adjustment, gain on sale of real estate, depreciable real estate reserves, loss on early extinguishment of debt, and gain (loss) on sale of marketable securities										186,920	7,256	90,289
Total assets	[1]	13,982,904				15,857,787				13,982,904	15,857,787
Marketing, general and administrative										100,498	99,759	94,873
Operating Segments | Real Estate Segment
Segment information
Total revenues										1,317,602	1,650,973	1,481,701
Income from continuing operations before equity in net gain on sale of interest in unconsolidated joint venture/real estate, purchase price fair value adjustment, gain on sale of real estate, depreciable real estate reserves, loss on early extinguishment of debt, and gain (loss) on sale of marketable securities										16,557	(197,000)	(71,634)
Total assets		11,696,560				13,868,672				11,696,560	13,868,672
Operating Segments | Debt and Preferred Equity Segment
Segment information
Total revenues										193,871	213,008	181,128
Income from continuing operations before equity in net gain on sale of interest in unconsolidated joint venture/real estate, purchase price fair value adjustment, gain on sale of real estate, depreciable real estate reserves, loss on early extinguishment of debt, and gain (loss) on sale of marketable securities										170,363	204,256	$ 161,923
Total assets		$ 2,286,344				$ 1,989,115				$ 2,286,344	$ 1,989,115

[1] The Company's consolidated balance sheets include assets and liabilities of consolidated variable interest entities ("VIEs"). See Note 2. The consolidated balance sheets include the following amounts related to our consolidated VIEs, excluding the Operating Partnership: $398.0 million and $412.3 million of land, $1.4 billion and $1.5 billion of building and improvements, $2.0 million and $2.0 million of building and leasehold improvements, $47.4 million and $47.4 million of properties under capital lease, $330.9 million and $327.2 million of accumulated depreciation, $221.0 million and $226.8 million of other assets included in other line items, $628.9 million and $621.8 million of real estate debt, net, $2.5 million and $2.2 million of accrued interest payable, $42.8 million and $42.1 million of capital lease obligations, and $56.8 million and $72.2 million of other liabilities included in other line items as of December 31, 2017 and December 31, 2016, respectively.