CONDENSED CONSOLIDATED STATEMENT OF CHANGES IN EQUITY (Unaudited) - USD ($) $ in Thousands	Total	Preferred Shares [Member]	Common Stock [Member]	Additional Paid-in-Capital [Member]	Accumulated Distributions in Excess of Net Income [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Non-controlling Interests [Member]
Beginning Balance at Dec. 31, 2014	$ 1,508,920	$ 94,016	$ 23	$ 2,763,374	$ (1,372,051)	$ 404	$ 23,154
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common shares upon conversion of convertible notes	3,630			3,630
Repurchase of common shares	(11,019)			(11,019)
Issuance of common shares and deferred compensation amortization, net	23,852		1	23,851
Acquisition of consolidated joint venture partners' equity interest	(1,234)				(1,247)		13
Dividends/distributions	(128,837)				(125,972)		(2,865)
Net income (loss)	79,378				76,853		2,525
Other comprehensive loss	(6,620)					(6,620)
Ending Balance at Sep. 30, 2015	1,468,070	94,016	24	2,779,836	(1,422,417)	(6,216)	22,827
Beginning Balance at Dec. 31, 2015	1,462,531	94,016	23	2,776,837	(1,428,908)	(1,939)	22,502
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common shares upon conversion of convertible notes	12,027			12,027
Repurchase of common shares	(8,973)			(8,973)
Redemption of noncontrolling OP units for common shares				31			(31)
Issuance of common shares and deferred compensation amortization, net	9,045		1	9,044
Dividends/distributions	(128,319)				(125,791)		(2,528)
Net income (loss)	72,875				73,215		(340)
Other comprehensive loss	(2,944)					(2,944)
Ending Balance at Sep. 30, 2016	$ 1,416,242	$ 94,016	$ 24	$ 2,788,966	$ (1,481,484)	$ (4,883)	$ 19,603