CONDENSED CONSOLIDATED STATEMENT OF CHANGES IN EQUITY - 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, 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	0			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)	79,521				79,623		(102)
Other comprehensive income	(2,944)					(2,944)
Ending Balance at Sep. 30, 2016	1,422,888	94,016	24	2,788,966	(1,475,076)	(4,883)	19,841
Beginning Balance at Dec. 31, 2016	1,412,491	94,016	24	2,800,736	(1,500,966)	(1,033)	19,714
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Redemption of noncontrolling OP units for common shares	0			574			(574)
Issuance of common shares and deferred compensation amortization, net	23,069			23,069
Dividends/distributions	(132,789)				(130,226)		(2,563)
Net income (loss)	55,181				54,733		448
Other comprehensive income	1,543					1,543
Ending Balance at Sep. 30, 2017	$ 1,359,495	$ 94,016	$ 24	$ 2,824,379	$ (1,576,459)	$ 510	$ 17,025