Consolidated Statements of Changes in Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in-Capital	Retained Earnings (Distributions in excess of net earnings)	Accumulated Other Comprehensive Income	Total Noncontrolling Interests	Operating Partnership	Consolidated Joint Venture
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 derivatives	(18,650)				(18,650)
Distribution 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
Balance (in shares) at Dec. 31, 2015		124,635,675
Balance at Dec. 31, 2015	2,200,524	$ 1,246	2,195,732	2,439	(16,602)	17,709	11,532	6,177
Increase (Decrease) in Owners' Equity
Net income	125,478			124,918		560	553	7
Unrealized loss on interest rate derivatives	(16,144)				(16,144)
Distribution to joint venture partner	(259)					(259)		(259)
Redemption of Operating Partnership units (in shares)		335,250
Redemption of Operating Partnership units	0	$ 3	4,322			(4,325)	(4,325)
Issuance of restricted stock (in shares)		581,544
Issuance of restricted stock	0	$ 6	(6)
Amortization of share-based compensation	3,935		3,935
Share grants to trustees (in shares)		2,554
Share grants to trustees	57		57
Shares acquired to satisfy minimum required federal and state tax withholding on vesting restricted stock (in shares)		(218,443)
Shares acquired to satisfy minimum required federal and state tax withholding on vesting restricted stock	(4,960)	$ (2)	(4,958)
Shares acquired as part of a share repurchase program (in shares)		(610,607)
Shares acquired as part of a share repurchase program	(13,271)	$ (6)	(13,265)
Forfeiture of restricted stock (in shares)		(426,310)
Forfeiture of restricted stock	0	$ (4)	4
Distributions on common shares and units	(123,969)			(123,417)		(552)	(552)
Balance (in shares) at Sep. 30, 2016		124,299,663
Balance at Sep. 30, 2016	$ 2,171,391	$ 1,243	$ 2,185,821	$ 3,940	$ (32,746)	$ 13,133	$ 7,208	$ 5,925