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			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	55,181				54,733		448
Other comprehensive loss	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
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	5,016			5,016
Repurchase of common shares	(17,387)			(17,387)
Repurchase of common shares to settle tax obligations	(2,544)			(2,544)
Forfeiture of employee common shares	(71)			(87)	16
Dividends/distributions	(133,667)				(130,968)		(2,699)
Net income	205,232				202,007		3,225
Other comprehensive loss	(413)					(413)
Ending Balance at Sep. 30, 2018	$ 1,397,001	$ 94,016	$ 24	$ 2,803,581	$ (1,518,669)	$ 652	$ 17,397