Agreements and Transactions with Related Parties - Related Party Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Amounts Included in the Consolidated Statements of Income
Available Cash Distributions	$ 5,185	$ 6,971	$ 11,355	$ 13,781
Personnel and overhead reimbursements	1,826	2,310	3,591	4,601
Interest expense on deferred acquisition fees	60	68	124	132
Director compensation	40	53	80	106
Operating expenses	14,604	16,741	30,135	33,284
Advisor Fees Capitalized
Current acquisition fees	127	3,537	130	3,823
Deferred acquisition fees	101	2,829	104	3,058
Personnel and overhead reimbursements	0	379	50	486
Transaction fees incurred	228	6,745	284	7,367
Asset management
Amounts Included in the Consolidated Statements of Income
Operating expenses	$ 7,493	$ 7,339	$ 14,985	$ 14,664