Related Party Transactions and Investments in Non-Consolidated Entities - Summarized financial information for Sponsored REITs (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) entity	Dec. 31, 2016 USD ($) entity	Dec. 31, 2015 USD ($) entity
Related Party Transactions and Investments in Non-Consolidated Entities
Number of REITs in which are included in the operations data | entity	7	9	10
Number of Sponsored REITs the Company held an interest in at period end | entity	6	7	9
Balance Sheet Data (unaudited):
Real estate, net	$ 312,861	$ 345,532
Other assets	74,076	86,594
Total liabilities	(151,092)	(164,820)
Shareholders' equity	235,845	267,306
Operating Data (unaudited):
Rental revenues	53,249	54,257	$ 57,777
Other revenues	7	39	34
Operating and maintenance expenses	(27,495)	(29,186)	(31,693)
Depreciation and amortization	(18,395)	(18,274)	(21,149)
Interest expense	(8,281)	(8,481)	(9,920)
Gain (loss) on sale, less applicable income tax	(702)	26,397
Net income (loss)	$ (1,617)	$ 24,752	$ (4,951)