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)	48,102			47,850		252	321	(69)
Unrealized loss on interest rate derivatives	(9,403)				(9,403)
Issuance of restricted stock (in shares)		253,242
Issuance of restricted stock		$ 3	(3)
Amortization of share-based compensation	4,023		4,023
Share grants to trustees (in shares)		1,057
Share grants to trustees	33		33
Shares acquired to satisfy minimum required federal and state tax withholding on vesting restricted stock (in shares)		(53,468)
Shares acquired to satisfy minimum required federal and state tax withholding on vesting restricted stock	(1,772)	$ (1)	(1,771)
Forfeiture of restricted stock (in shares)		(229)
Distributions on common shares and units	(44,240)			(43,945)		(295)	(295)
Balance (in shares) at Mar. 31, 2015		132,165,308
Balance at Mar. 31, 2015	2,375,227	$ 1,321	2,422,013	(42,510)	(23,047)	17,450	11,224	6,226
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)	25,350			25,298		52	114	(62)
Unrealized loss on interest rate derivatives	(20,248)				(20,248)
Distributions 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)		378,567
Issuance of restricted stock	0	$ 4	(4)
Amortization of share-based compensation	2,591		2,591
Share grants to trustees (in shares)		1,447
Share grants to trustees	33		33
Shares acquired to satisfy minimum required federal and state tax withholding on vesting restricted stock (in shares)		(32,398)
Shares acquired to satisfy minimum required federal and state tax withholding on vesting restricted stock	(653)		(653)
Shares acquired as part of a share repurchase program (in shares)		(510,498)
Shares acquired as part of a share repurchase program	(11,289)	$ (5)	(11,284)
Forfeiture of restricted stock (in shares)		(263)
Distributions on common shares and units	(41,367)			(41,185)		(182)	(182)
Balance (in shares) at Mar. 31, 2016		124,807,780
Balance at Mar. 31, 2016	$ 2,154,682	$ 1,248	$ 2,190,737	$ (13,448)	$ (36,850)	$ 12,995	$ 7,139	$ 5,856