Consolidated Statements of Changes in Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in-Capital [Member]	Retained Earnings (Distributions in excess of net earnings) [Member]	Accumulated Other Comprehensive Loss [Member]	Total Noncontrolling Interests [Member]	Operating Partnership [Member]	Consolidated Joint Venture [Member]
Balance (in shares) at Dec. 31, 2012		106,565,516
Balance at Dec. 31, 2012	$ 1,807,911	$ 1,066	$ 1,841,449	$ (52,681)	$ 0	$ 18,077	$ 11,311	$ 6,766
Increase (Decrease) in Equity
Net income (loss)	114,179			112,921		1,258	718	540
Unrealized gain (loss) on interest rate derivatives	(5,941)				(5,941)
Proceeds from sale of common stock, net (in shares)		15,870,000
Proceeds from sale of common stock, net	327,547	$ 159	327,388
Issuance of restricted stock (in shares)		377,830
Issuance of restricted stock	0	$ 3	(3)
Amortization of share based compensation	13,078		13,078
Share grants to trustees (in shares)		5,357
Share grants to trustees	124		124
Shares acquired to satisfy minimum required federal and state tax withholding on vesting restricted stock (in shares)		(173,409)
Shares acquired to satisfy statutory minimum federal and state tax obligations on vesting restricted stock	(4,034)	$ (2)	(4,032)
Forfeiture of restricted stock (in shares)		(5,252)
Distributions on common shares and units	(106,530)			(105,762)		(768)	(768)
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 Equity
Net income (loss)	136,480			135,441		1,039	868	171
Unrealized gain (loss) on interest rate derivatives	(7,703)				(7,703)
Proceeds from sale of common stock, net (in shares)		9,200,000
Proceeds from sale of common stock, net	232,712	$ 92	232,620
Issuance of restricted stock (in shares)		343,887
Issuance of restricted stock	0	$ 3	(3)
Amortization of share based compensation	15,088		15,088
Share grants to trustees (in shares)		4,347
Share grants to trustees	127		127
Shares acquired to satisfy minimum required federal and state tax withholding on vesting restricted stock (in shares)		(211,983)
Shares acquired to satisfy statutory minimum federal and state tax obligations on vesting restricted stock	(6,107)	$ (2)	(6,105)
Forfeiture of restricted stock (in shares)		(11,587)
Distribution to joint venture partner	(1,182)					(1,182)		(1,182)
Distributions on common shares and units	(137,265)			(136,334)		(931)	(931)
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 Equity
Net income (loss)	219,812			218,221		1,591	1,514	77
Unrealized gain (loss) on interest rate derivatives	(2,958)				(2,958)
Shares acquired as part of a share repurchase program (in shares)		(8,044,372)
Shares acquired as part of a share repurchase program	(225,186)	$ (80)	(225,106)
Issuance of restricted stock (in shares)		1,126,431
Issuance of restricted stock		$ 11	(11)
Amortization of share based compensation	13,002		13,002
Share grants to trustees (in shares)		5,008
Share grants to trustees	132		132
Shares acquired to satisfy minimum required federal and state tax withholding on vesting restricted stock (in shares)		(403,722)
Shares acquired to satisfy statutory minimum federal and state tax obligations on vesting restricted stock	(12,020)	$ (4)	(12,016)
Forfeiture of restricted stock (in shares)		(12,376)
Distribution to joint venture partner	(195)					(195)		(195)
Distributions on common shares and units	(170,547)			(169,367)		(1,180)	(1,180)
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