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, 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			485			(485)
Issuance of common shares and deferred compensation amortization, net	20,996		0	20,996
Dividends/distributions	(88,423)				(86,668)		(1,755)
Net income (loss)	49,585				49,192		393
Other comprehensive income	1,476					1,476
Ending Balance at Jun. 30, 2017	1,396,125	94,016	24	2,822,217	(1,538,442)	443	17,867
Beginning Balance at Dec. 31, 2017	1,340,835	94,016	24	2,818,520	(1,589,724)	1,065	16,934
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Redemption of noncontrolling OP units for common shares	0			63			(63)
Issuance of common shares and deferred compensation amortization, net	3,391			3,391
Repurchase of common shares	(7,362)			(7,362)
Repurchase of common shares to settle tax obligations	(2,544)			(2,544)
Forfeiture of employee common shares	(78)			(87)	9
Dividends/distributions	(89,361)				(87,441)		(1,920)
Net income (loss)	(15,618)				(16,009)		391
Other comprehensive income	33					33
Ending Balance at Jun. 30, 2018	$ 1,229,296	$ 94,016	$ 24	$ 2,811,981	$ (1,693,165)	$ 1,098	$ 15,342