Consolidated Statements of Beneficiaries' Equity - USD ($) $ in Thousands	Total	Preferred Stock [Member]	Common Stock [Member]	Deferred Compensation, Share-based Payments [Member]	Additional Paid-in Capital [Member]	Common Stock In Grantor Trust [Member]	Cumulative Earnings [Member]	Accumulated Other Comprehensive Loss [Member]	Cumulative Distributions [Member]	Noncontrolling Interest [Member]
Beginning Balance, Shares at Dec. 31, 2015		4,000,000	174,688,568	745,686
Beginning Balance at Dec. 31, 2015	$ 1,952,091	$ 40	$ 1,747	$ 11,918	$ 3,252,622	$ (11,918)	$ 499,086	$ (5,192)	$ (1,814,378)	$ 18,166
Net income	46,310						45,921			389
Other comprehensive income (loss)	(9,159)							(9,079)		(80)
Issuance of partnership interest in consolidated real estate venture	18									18
Share-based compensation activity, Shares			210,116
Share-based compensation activity	3,332		$ 2		3,296		34
Share Issuance from/(to) Deferred Compensation Plan, Shares			(8,252)	101,321
Share Issuance from/(to) Deferred Compensation Plan				$ 1,237		(1,237)
Adjustment to Non-controlling Interest					(10)					10
Preferred Share distributions	(1,725)								(1,725)
Distributions declared	(26,577)								(26,347)	(230)
Ending Balance, Shares at Mar. 31, 2016		4,000,000	174,890,432	847,007
Ending Balance at Mar. 31, 2016	1,964,290	$ 40	$ 1,749	$ 13,155	3,255,908	(13,155)	545,041	(14,271)	(1,842,450)	18,273
Beginning Balance, Shares at Dec. 31, 2016		4,000,000	175,140,760	899,457
Beginning Balance at Dec. 31, 2016	1,883,437	$ 40	$ 1,752	$ 13,684	3,258,870	(13,684)	539,319	(1,745)	(1,931,892)	17,093
Net income	21,271						21,102			169
Other comprehensive income (loss)	1,300							1,288		12
Issuance of partnership interest in consolidated real estate venture	29									29
Equity issuance costs	(219)				(219)
Bonus share issuance, Shares			6,752
Bonus share issuance	110				110
Share-based compensation activity, Shares			56,669	39,870
Share-based compensation activity	3,770				3,769		1
Share Issuance from/(to) Deferred Compensation Plan, Shares			(354)	(2,388)
Share Issuance from/(to) Deferred Compensation Plan	(48)			$ 560	(48)	(560)
Share Choice Plan Issuance, Shares			(1,423)
Adjustment to Non-controlling Interest					(23)					23
Preferred Share distributions	(1,725)								(1,725)
Distributions declared	(28,359)								(28,122)	(237)
Ending Balance, Shares at Mar. 31, 2017		4,000,000	175,202,404	936,939
Ending Balance at Mar. 31, 2017	$ 1,879,566	$ 40	$ 1,752	$ 14,244	$ 3,262,459	$ (14,244)	$ 560,422	$ (457)	$ (1,961,739)	$ 17,089