Equity Investments - Summary of Unaudited Income Statement Information of Equity Method Investee (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Income Statements:
Rental income	$ 163,697	$ 172,407	$ 331,669	$ 344,447
Tenant reimbursements and other income	39,997	42,787	85,080	88,007
Total revenues	203,694	215,194	416,749	432,454
Operating expenses	89,686	92,701	187,557	194,432
General and administrative	10,911	24,097	27,469	48,945
Total expenses	169,492	199,312	348,524	372,784
Operating income	34,202	15,882	68,225	59,670
Interest expense	(27,973)	(37,899)	(57,815)	(75,834)
Gain on early extinguishment of debt	10,426	0	9,998	0
Income before income tax expense and equity in earnings of an investee	15,531	(4,858)	28,600	1,488
Income tax expense	(2,915)	(908)	(2,354)	(1,463)
Equity in earnings of investees	0	12,454	0	23,388
Net income	$ 12,616	$ 5,385	$ 26,246	$ 25,833
Weighted average common shares outstanding (in shares)	129,733	123,812	129,714	121,121
SIR
Income Statements:
Rental income		$ 48,465		$ 93,528
Tenant reimbursements and other income		8,092		16,057
Total revenues		56,557		109,585
Operating expenses		9,985		19,964
Depreciation and amortization		10,495		19,789
Acquisition related costs		136		374
General and administrative		2,198		7,374
Total expenses		22,814		47,501
Operating income		33,743		62,084
Interest expense		(3,634)		(6,992)
Gain on early extinguishment of debt		0		243
Income before income tax expense and equity in earnings of an investee		30,109		55,335
Income tax expense		(19)		(90)
Equity in earnings of investees		118		21
Net income		$ 30,208		$ 55,266
Weighted average common shares outstanding (in shares)		54,178		52,021
Net income per common share (in dollars per share)		$ 0.56		$ 1.06