Related Party Transactions and Investments in Non-Consolidated Entities - Summarized financial information for Sponsored REITs (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($) entity	Jun. 30, 2017 USD ($) entity	Dec. 31, 2017 USD ($) entity
Related Party Transactions and Investments in Non-Consolidated Entities
Number of REITs in which are included in the operations data | entity	6	7
Number of Sponsored REITs the Company held an interest in at period end | entity	5		6
Balance Sheet Data (unaudited):
Real estate, net	$ 296,588		$ 312,861
Other assets	69,439		74,076
Total liabilities	(148,896)		(151,092)
Shareholders' equity	217,131		$ 235,845
Operating Data (unaudited):
Rental revenues	25,755	$ 28,084
Other revenues	1	4
Operating and maintenance expenses	(13,175)	(14,466)
Depreciation and amortization	(8,606)	(9,943)
Interest expense	(4,030)	(4,226)
Gain (loss) on sale, less applicable income tax	9,393
Net income (loss)	$ 9,338	$ (547)