Consolidated Statements of Beneficiaries' Equity - USD ($) $ in Thousands	Total	Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Deferred Compensation, Share-based Payments [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	$ 3,314,693	$ 6,219	$ (6,219)	$ 529,487	$ (4,607)	$ (1,700,579)	$ 18,499
Net income	8,594						8,536			58
Other comprehensive loss	(2,605)							(2,583)		(22)
Bonus share issuance, Shares			8,447
Bonus share issuance	125			125
Equity issuance costs	(48)			(48)
Share-based compensation activity, Shares			410,506		280,011
Share-based compensation activity	2,352		$ 5	2,347
Share Issuance from/to Deferred Compensation Plan, Shares			33,485		30,144
Share Issuance from/to Deferred Compensation Plan	0				$ 4,975	(4,975)
Adjustment to Non-controlling Interest	0			20						(20)
Distribution to preferred shareholders	(1,725)								(1,725)
Distributions declared	(27,443)								(27,213)	(230)
Ending Balance, Shares at Mar. 31, 2015		4,000,000	179,745,598		694,691
Ending Balance at Mar. 31, 2015	2,138,576	$ 40	$ 1,798	3,317,137	$ 11,194	(11,194)	538,023	(7,190)	(1,729,517)	18,285
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	3,252,622	$ 11,918	(11,918)	499,086	(5,192)	(1,814,378)	18,166
Net income	46,310						45,921			389
Other comprehensive 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	0				$ 1,237	(1,237)
Adjustment to Non-controlling Interest	0			(10)						10
Distribution to preferred shareholders	(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	$ 3,255,908	$ 13,155	$ (13,155)	$ 545,041	$ (14,271)	$ (1,842,450)	$ 18,273