Agreements and Transactions with Related Parties - Related Party Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Amounts Included in the Consolidated Statements of Income
Asset management fees	$ 29,363	$ 29,705	$ 29,192
Available Cash Distributions	26,675	24,765	24,668
Personnel and overhead reimbursements	8,878	9,684	12,199
Director compensation	310	310	310
Interest expense on deferred acquisition fees and loan from affiliate	273	238	309
Acquisition expenses	0	2,844	430
Operating expenses	65,499	67,546	67,108
Acquisition Fees Capitalized
Current acquisition fees	5,284	3,985	8,180
Deferred acquisition fees	4,228	3,188	6,325
Personnel and overhead reimbursements	849	584	858
Transaction fees incurred	$ 10,361	$ 7,757	$ 15,363