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, 2014		4,000,000	179,293,160	384,536
Beginning Balance at Dec. 31, 2014	$ 2,159,326	$ 40	$ 1,793	$ 6,219	$ 3,314,693	$ (6,219)	$ 529,487	$ (4,607)	$ (1,700,579)	$ 18,499
Net income	11,652						11,592			60
Other comprehensive loss	(1,053)							(1,044)		(9)
Issuance of partnership interest in joint venture	1,025									1,025
Bonus share issuance, Shares			8,447
Bonus share issuance	125				125
Equity issuance costs	(48)				(48)
Share-based compensation activity, Shares			509,675	280,011
Share-based compensation activity	2,955		$ 6		2,949
Share Issuance from/to Deferred Compensation Plan, Shares			86,839	83,498
Share Issuance from/to Deferred Compensation Plan	0			$ 5,777		(5,777)
Adjustment to Non-controlling Interest	0				32					(32)
Distribution to preferred shareholders	(3,450)								(3,450)
Distributions declared	(54,726)								(54,265)	(461)
Ending Balance, Shares at Jun. 30, 2015		4,000,000	179,898,121	748,045
Ending Balance at Jun. 30, 2015	2,115,806	$ 40	$ 1,799	$ 11,996	3,317,751	(11,996)	541,079	(5,651)	(1,758,294)	19,082
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	44,987						44,620			367
Other comprehensive loss	(12,686)							(12,577)		(109)
Issuance of partnership interest in consolidated real estate venture	54									54
Conversion of LP Units to Common Shares, Shares			55,303
Conversion of LP Units to Common Shares	0		$ 1		874					(875)
Share-based compensation activity, Shares			365,414
Share-based compensation activity	3,615		$ 3		3,575		37
Share Issuance from/to Deferred Compensation Plan, Shares			(8,252)	155,583
Share Issuance from/to Deferred Compensation Plan	(46)			$ 1,826	(46)	(1,826)
Adjustment to Non-controlling Interest	0				(290)					290
Distribution to preferred shareholders	(3,450)								(3,450)
Distributions declared	(54,739)								(54,272)	(467)
Ending Balance, Shares at Jun. 30, 2016		4,000,000	175,101,033	901,269
Ending Balance at Jun. 30, 2016	$ 1,929,826	$ 40	$ 1,751	$ 13,744	$ 3,256,735	$ (13,744)	$ 543,743	$ (17,769)	$ (1,872,100)	$ 17,426