Real Estate and Real Estate-Related Investments - Sales of Real Estate Properties Included in Net Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Real Estate Properties [Line Items]
Net income attributable to the Company related to dispositions	$ 10,846	$ 16,107	$ (1,539)	$ (856)	$ (2,202)	$ 9,383	$ (1,165)	$ (1,073)	$ 24,558	$ 4,943	$ 6,978
Sale of Real Estate | Lakewood Flats, Babcock, AJS, Holstenplatz, Wimberly, and 1875 Lawrence
Real Estate Properties [Line Items]
Net income attributable to the Company related to dispositions										$ 16,900	$ 12,400	$ 17,400