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, 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 (loss)	104,512			103,841		671	694	(23)
Unrealized loss on interest rate derivatives	(3,484)
Distribution to joint venture partner	(195)					(195)		(195)
Issuance of restricted stock (in shares)		287,497
Issuance of restricted stock	0	$ 3	(3)
Amortization of share-based compensation	7,791		7,791
Share grants to trustees (in shares)		2,168
Share grants to trustees	66		66
Shares acquired to satisfy minimum required federal and state tax withholding on vesting restricted stock (in shares)		(120,249)
Shares acquired to satisfy minimum required federal and state tax withholding on vesting restricted stock	(3,800)	$ (1)	(3,799)
Shares acquired as part of a share repurchase program (in shares)		(1,995,177)
Shares acquired as part of a share repurchase program	(59,841)	$ (20)	(59,821)
Forfeiture of restricted stock (in shares)		(5,327)
Distributions on common shares and units	(87,809)			(87,219)		(590)	(590)
Balance (in shares) at Jun. 30, 2015		130,133,618
Balance at Jun. 30, 2015	2,335,724	$ 1,301	2,363,965	(29,793)	(17,128)	17,379	11,302	6,077
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 (loss)	84,089			83,744		345	370	(25)
Unrealized loss on interest rate derivatives	(25,614)
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)		439,996
Issuance of restricted stock	0	$ 4	(4)
Amortization of share-based compensation	2,014		2,014
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)		(54,566)
Shares acquired to satisfy minimum required federal and state tax withholding on vesting restricted stock	(1,117)		(1,117)
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)		(419,656)
Forfeiture of restricted stock	0	$ (4)	4
Distributions on common shares and units	(82,672)			(82,306)		(366)	(366)
Balance (in shares) at Jun. 30, 2016		124,328,646
Balance at Jun. 30, 2016	$ 2,163,751	$ 1,243	$ 2,187,743	$ 3,877	$ (42,216)	$ 13,104	$ 7,211	$ 5,893