Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Information.
Rental income	$ 64,625	$ 64,158	$ 62,428	$ 59,820	$ 58,271	$ 56,401	$ 55,934	$ 56,304	$ 251,031	$ 226,910	$ 203,700
Expenses:
Real Estate Tax Expense									28,389	25,710	22,485
Utility expenses									19,369	17,116	15,767
Other operating expenses									45,982	41,134	37,074
Depreciation and amortization									66,593	55,699	49,070
Loss on asset impairment									2,016
Acquisition related costs									1,344	2,439	1,614
General and administrative									15,809	12,710	11,924
Total expenses									179,502	154,808	137,934
Operating Income (Loss)									71,529	72,102	65,766
Investment Income, Interest									69	37	29
Interest expense									(28,048)	(16,831)	(16,892)
Loss on early extinguishment of debt									(1,307)
Loss on issuance of shares by an equity investee									(53)
Income from continuing operations before income tax expense and equity in earnings of investees									42,243	55,308	48,903
Income tax expense									(117)	(133)	(159)
Equity in earnings of investees									10,963	334	316
Income from continuing operations									53,036	55,509	49,060
Income (loss) from discontinued operations									3,498	(889)	900
Net income	14,114	12,622	14,608	15,190	12,724	1,966	15,204	24,726	56,534	54,620	49,960
Total assets	2,427,615				1,632,452				2,427,615	1,632,452
Operating Segments | Investment in Real Estate
Segment Information.
Rental income									251,031
Expenses:
Real Estate Tax Expense									28,389
Utility expenses									19,369
Other operating expenses									45,982
Depreciation and amortization									66,593
Loss on asset impairment									2,016
Acquisition related costs									1,344
Total expenses									163,693
Operating Income (Loss)									87,338
Interest expense									(7,820)
Income from continuing operations before income tax expense and equity in earnings of investees									79,518
Income from continuing operations									79,518
Income (loss) from discontinued operations									3,498
Net income									83,016
Total assets	1,714,130								1,714,130
Operating Segments | Investment in SIR
Expenses:
Equity in earnings of investees									10,876
Income from continuing operations									10,823
Net income									10,823
Total assets	680,137								680,137
Corporate, Non-Segment
Expenses:
General and administrative									15,809
Total expenses									15,809
Operating Income (Loss)									(15,809)
Investment Income, Interest									69
Interest expense									(20,228)
Loss on early extinguishment of debt									(1,307)
Income from continuing operations before income tax expense and equity in earnings of investees									(37,275)
Income tax expense									(117)
Equity in earnings of investees									87
Income from continuing operations									(37,305)
Net income									(37,305)
Total assets	$ 33,348								$ 33,348