Consolidated Statements of Changes in Equity (USD $) In Thousands, except Share data, unless otherwise specified	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 at Dec. 31, 2013	$ 2,146,334	$ 1,226	$ 2,178,004	$ (45,522)	$ (5,941)	$ 18,567	$ 11,261	$ 7,306
Balance (in shares) at Dec. 31, 2013		122,640,042
Increase (Decrease) in Owners' Equity
Net income (loss)	11,985			11,932		53	87	(34)
Sale of common stock (shares)		0
Proceeds from sale of common stock		0	0
Unrealized loss on interest rate derivative	(1,361)
Distributions to joint venture partner	(1,182)
Issuance of restricted stock		3	(3)
Issuance of restricted stock (in shares)		305,053
Amortization of share based compensation	3,573		3,573
Share grants to trustees	28		28
Share grants to trustees (in shares)		1,051
Shares acquired to satisfy minimum required federal and state tax withholding on vesting restricted stock	(1,097)	0	(1,097)
Shares acquired to satisfy minimum required federal and state tax withholding on vesting restricted stock (in shares)		(42,900)
Forfeiture of restricted stock (in shares)		(1,905)
Distributions on common shares and units	(27,453)			(27,258)		(195)	(195)
Balance at Mar. 31, 2014	2,130,827	1,229	2,180,505	(60,848)	(7,302)	17,243	11,153	6,090
Balance (in shares) at Mar. 31, 2014		122,901,341
Balance at Dec. 31, 2014	2,378,484	1,319	2,419,731	(46,415)	(13,644)	17,493	11,198	6,295
Balance (in shares) at Dec. 31, 2014		131,964,706
Increase (Decrease) in Owners' Equity
Net income (loss)	48,102			47,850		252	321	(69)
Unrealized loss on interest rate derivative	(9,403)				(9,403)
Issuance of restricted stock		3	(3)
Issuance of restricted stock (in shares)		253,242
Amortization of share based compensation	4,023		4,023
Share grants to trustees	33		33
Share grants to trustees (in shares)		1,057
Shares acquired to satisfy minimum required federal and state tax withholding on vesting restricted stock	(1,772)	(1)	(1,771)
Shares acquired to satisfy minimum required federal and state tax withholding on vesting restricted stock (in shares)		(53,468)
Forfeiture of restricted stock (in shares)		(229)
Distributions on common shares and units	(44,240)			(43,945)		(295)	(295)
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 Mar. 31, 2015		132,165,308