Segment Information (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2017 USD ($) segment	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Number of segments | segment	2
Rental income	$ 69,296	$ 63,611
Expenses:
Real estate taxes	8,177	7,653
Utility expenses	4,606	4,174
Other operating expenses	13,992	12,911
Depreciation and amortization	20,505	18,324
Acquisition related costs	0	152
General and administrative	3,962	3,526
Total expenses	51,242	46,740
Operating income	18,054	16,871
Dividend income	304	0
Interest income	61	6
Interest expense	(13,581)	(9,364)
Gain on early extinguishment of debt	0	104
Income from continuing operations before income taxes and equity in earnings of investees	4,838	7,617
Income tax expense	(18)	(15)
Equity in earnings of investees	2,739	9,934
Income from continuing operations	7,559	17,536
Loss from discontinued operations	(144)	(149)
Net income	7,415	17,387
Total assets	2,377,050		$ 2,385,066
Operating Segments | Investment in Real Estate
Segment Reporting Information [Line Items]
Rental income	69,296	63,611
Expenses:
Real estate taxes	8,177	7,653
Utility expenses	4,606	4,174
Other operating expenses	13,992	12,911
Depreciation and amortization	20,505	18,324
Acquisition related costs		152
Total expenses	47,280	43,214
Operating income	22,016	20,397
Interest income	46
Interest expense	(432)	(1,095)
Gain on early extinguishment of debt		104
Income from continuing operations before income taxes and equity in earnings of investees	21,630	19,406
Income from continuing operations	21,630	19,406
Loss from discontinued operations	(144)	(149)
Net income	21,486	19,257
Total assets	1,811,545		1,807,560
Operating Segments | Investment in SIR
Expenses:
Equity in earnings of investees	2,611	9,857
Income from continuing operations	2,611	9,857
Net income	2,611	9,857
Total assets	482,103		487,708
Corporate, Non-Segment
Expenses:
General and administrative	3,962	3,526
Total expenses	3,962	3,526
Operating income	(3,962)	(3,526)
Dividend income	304
Interest income	15	6
Interest expense	(13,149)	(8,269)
Income from continuing operations before income taxes and equity in earnings of investees	(16,792)	(11,789)
Income tax expense	(18)	(15)
Equity in earnings of investees	128	77
Income from continuing operations	(16,682)	(11,727)
Net income	(16,682)	$ (11,727)
Total assets	$ 83,402		$ 89,798