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, 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 gain on interest rate derivatives	(20,248)				(20,248)
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)		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)	$ 0	(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)
Forfeiture of restricted stock	0
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
Balance (in shares) at Dec. 31, 2016		124,364,178
Balance at Dec. 31, 2016	2,235,277	$ 1,244	2,187,333	38,249	(4,902)	13,353	7,380	5,973
Increase (Decrease) in Owners' Equity
Net income (loss)	21,777			21,758		19	85	(66)
Unrealized gain on interest rate derivatives	5,548				5,548
Issuance of restricted stock (in shares)		264,303
Issuance of restricted stock	0	$ 2	(2)
Amortization of share-based compensation	2,334		2,334
Shares acquired to satisfy minimum required federal and state tax withholding on vesting restricted stock (in shares)		(20,861)
Shares acquired to satisfy minimum required federal and state tax withholding on vesting restricted stock	(486)	$ 0	(486)
Forfeiture of restricted stock (in shares)		(131)
Forfeiture of restricted stock	0	$ 0	0
Distributions on common shares and units	(41,480)			(41,296)		(184)	(184)
Balance (in shares) at Mar. 31, 2017		124,607,489
Balance at Mar. 31, 2017	$ 2,222,970	$ 1,246	$ 2,189,179	$ 18,711	$ 646	$ 13,188	$ 7,281	$ 5,907