CONSOLIDATED STATEMENTS OF EQUITY - USD ($) $ in Thousands	Total	Shares of Beneficial Interest, $1.00 Par [Member]	Capital Contributed in Excess of Par [Member]	Accumulated Other Comprehensive (Income) Loss [Member]	Distributions in Excess of Net Income [Member]	Non- controlling interest [Member]	Series B Preferred Stock [Member]	Series B Preferred Stock [Member] Distributions in Excess of Net Income [Member]	Series C Preferred Stock [Member]	Series C Preferred Stock [Member] Distributions in Excess of Net Income [Member]	Series D Preferred Stock [Member]	Series D Preferred Stock [Member] Distributions in Excess of Net Income [Member]	Series A Preferred Stock [Member]	Series A Preferred Stock [Member] Distributions in Excess of Net Income [Member]
Preferred Stock, Dividends, Per Share, Cash Paid							$ 1.8438						$ 2.0625
Beginning Balance at Dec. 31, 2014	$ 844,737	$ 68,801	$ 1,474,183	$ (6,002)	$ (721,605)	$ 29,279	$ 35						$ 46
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(129,567)				(116,683)	(12,884)
Other comprehensive income	2,027			1,809		218
Shares issued upon redemption of Operating Partnership units	0	(34)	(675)			709
Shares issued under employee and trustee compensation plans, net of shares retired	(4,383)	362	(4,745)
Amortization of deferred compensation	6,284		6,284
Distributions paid to common shareholders	(58,085)				(58,085)
Distributions paid to preferred shareholders	(15,848)						(6,361)						(9,487)
Noncontrolling interests:
Distributions paid to Operating Partnership unit holders	(5,703)					(5,703)
Operating Partnership Units Issued As Consideration For Acquisition	145,188					145,188
Contributions from noncontrolling interest, net	(20)					(20)
Ending Balance at Dec. 31, 2015	784,630	69,197	1,476,397	(4,193)	(912,221)	155,369	$ 35						$ 46
Preferred Stock, Dividends, Per Share, Cash Paid							$ 1.8438						$ 2.0625
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(12,713)				(11,348)	(1,365)
Other comprehensive income	6,510			5,815		695
Shares issued upon redemption of Operating Partnership units	0	(26)	(574)			600
Shares issued under employee and trustee compensation plans, net of shares retired	(889)	330	(1,219)
Amortization of deferred compensation	6,035		6,035
Distributions paid to common shareholders	(58,372)				(58,372)
Distributions paid to preferred shareholders	(15,848)						$ (6,361)						$ (9,487)	$ (9,487)
Noncontrolling interests:
Distributions paid to Operating Partnership unit holders	(6,991)					(6,991)
Contributions from noncontrolling interest, net	44					44
Ending Balance at Dec. 31, 2016	702,406	69,553	1,481,787	1,622	(997,789)	147,152	$ 35		$ 0		$ 0		$ 46
Preferred Stock, Dividends, Per Share, Cash Paid							$ 1.8438		$ 1.5900		$ 0.448785		$ 1.701575
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(32,848)				(29,344)	(3,504)
Other comprehensive income	6,274			5,604		670
Stock Issued During Period, Value, New Issues	286,848		286,729						$ 69		$ 50
Proceeds from (Repurchase of) Redeemable Preferred Stock	(115,000)		(110,851)										$ (46)
Shares issued upon redemption of Operating Partnership units		(39)	(375)			(414)
Shares issued under employee and trustee compensation plans, net of shares retired	608	391	217
Amortization of deferred compensation	5,709		5,709
Distributions paid to common shareholders	(58,651)				(58,651)
Distributions paid to preferred shareholders	(27,845)						$ (6,361)	$ (6,361)	(10,971)	$ (10,971)	(2,244)	$ (2,244)	(7,827)	$ (7,827)
Noncontrolling interests:
Distributions paid to Operating Partnership unit holders	(6,970)					(6,970)
Contributions from noncontrolling interest, net	18					18
Ending Balance at Dec. 31, 2017	$ 760,991	$ 69,983	$ 1,663,966	$ 7,226	$ (1,117,290)	$ 136,952	$ 35		$ 69		$ 50		$ 0