Fair Value Measurements - Nonrecurring Measurements (Details) - Nonrecurring - Level 3 - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Impairment Charges From Continuing Operations
Fair Value Measurements				$ 4,700
Impairment charges and other credit losses	$ 6,168	$ 2,510	$ 11,572	7,029
Net investments in direct financing leases
Impairment Charges From Continuing Operations
Fair Value Measurements	13,597	0	13,597	0
Impairment charges and other credit losses	6,168	0	6,168	0
CMBS
Impairment Charges From Continuing Operations
Fair Value Measurements			1,144	0
Impairment charges and other credit losses			5,404	0
Equity investments in real estate
Impairment Charges From Continuing Operations
Fair Value Measurements	0	4,780	0	4,780
Impairment charges and other credit losses	$ 0	$ 2,510	0	2,510
Real estate
Impairment Charges From Continuing Operations
Fair Value Measurements			0	4,719
Impairment charges and other credit losses			$ 0	$ 4,519