Fair Value Measurements - Nonrecurring Measurements (Details) - USD ($) $ in Thousands	3 Months Ended				12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Sep. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Impairment Charges From Continuing Operations
Impairment charges				$ 29,200
Nonrecurring | Level 3
Impairment Charges From Continuing Operations
Impairment charges					$ 19,535	$ 31,625	$ 1,023
Equity investments in real estate | Nonrecurring | Level 3
Impairment Charges From Continuing Operations
Fair Value Measurements					10,242	12,528	0
Impairment charges		$ 6,300	$ 2,500		10,576	1,919	0
Real estate | Nonrecurring | Level 3
Impairment Charges From Continuing Operations
Fair Value Measurements					7,525	14,850	0
Impairment charges	$ 4,200				8,276	29,183	0
CMBS | Nonrecurring | Level 3
Impairment Charges From Continuing Operations
Fair Value Measurements					258	400	1,478
Impairment charges					$ 683	$ 523	$ 1,023