Consolidated Statements of Changes in Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in-Capital	Distributions in excess of net earnings	Accumulated Other Comprehensive Income	Total Noncontrolling Interests	Operating Partnership	Consolidated Joint Venture
Balance (in shares) at Dec. 31, 2013		122,640,042
Balance at Dec. 31, 2013	$ 2,146,334	$ 1,226	$ 2,178,004	$ (45,522)	$ (5,941)	$ 18,567	$ 11,261	$ 7,306
Increase (Decrease) in Owners' Equity
Net income	102,410			101,596		814	712	102
Proceeds from sale of common stock, net (shares)		9,200,000
Proceeds from sale of common stock, net	232,756	$ 92	232,664
Unrealized loss on interest rate derivative	(1,346)				(1,346)
Distributions to joint venture partner	(1,182)					(1,182)		(1,182)
Issuance of restricted stock (in shares)		343,887
Issuance of restricted stock		$ 3	(3)
Amortization of share-based compensation	11,244		11,244
Share grants to trustees (in shares)		3,360
Share grants to trustees	94		94
Shares acquired to satisfy minimum required federal and state tax withholding on vesting restricted stock (in shares)		(154,277)
Shares acquired to satisfy minimum required federal and state tax withholding on vesting restricted stock	(4,246)	$ (2)	(4,244)
Forfeiture of restricted stock (in shares)		(8,716)
Distributions on common shares and units	(97,107)			(96,443)		(664)	(664)
Balance (in shares) at Sep. 30, 2014		132,024,296
Balance at Sep. 30, 2014	2,388,957	$ 1,319	2,417,759	(40,369)	(7,287)	17,535	11,309	6,226
Balance (in shares) at Dec. 31, 2014		131,964,706
Balance at Dec. 31, 2014	2,378,484	$ 1,319	2,419,731	(46,415)	(13,644)	17,493	11,198	6,295
Increase (Decrease) in Owners' Equity
Net income	145,445			144,435		1,010	984	26
Unrealized loss on interest rate derivative	(18,650)				(18,650)
Distributions to joint venture partner	(195)					(195)		(195)
Issuance of restricted stock (in shares)		1,126,431
Issuance of restricted stock	0	$ 11	(11)
Amortization of share-based compensation	10,488		10,488
Share grants to trustees (in shares)		3,477
Share grants to trustees	99		99
Shares acquired to satisfy minimum required federal and state tax withholding on vesting restricted stock (in shares)		(363,512)
Shares acquired to satisfy minimum required federal and state tax withholding on vesting restricted stock	(11,029)	$ (3)	(11,026)
Shares acquired as part of a share repurchase program (in shares)		(6,992,708)
Shares acquired as part of a share repurchase program	(199,944)	$ (70)	(199,874)
Forfeiture of restricted stock (in shares)		(12,376)
Distributions on common shares and units	(129,122)			(128,237)		(885)	(885)
Balance (in shares) at Sep. 30, 2015		125,726,018
Balance at Sep. 30, 2015	$ 2,175,576	$ 1,257	$ 2,219,407	$ (30,217)	$ (32,294)	$ 17,423	$ 11,297	$ 6,126