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 Income (Loss) [Member]	Cumulative Distributions [Member]	Noncontrolling Interest [Member]
Beginning Balance at Dec. 31, 2015	$ 1,952,091	$ 40	$ 1,747	$ 11,918	$ 3,252,622			$ (5,192)	$ (1,814,378)	$ 18,166
Beginning Balance, Shares at Dec. 31, 2015		4,000,000	174,688,568	745,686		(11,918)	499,086
Net income	40,501						$ 40,191			310
Other comprehensive income	3,475							3,447		28
Issuance of partnership interest in consolidated real estate venture	108									108
Conversion of LP Units to Common Shares	$ 0		$ 1		874					(875)
Conversion of LP Units to Common Shares, Shares			55,303
Repurchase and retirement of Common Shares of Beneficial Interest, Shares	0
Share-based compensation activity	$ 5,764		$ 4		5,718		42
Share-based compensation activity, Shares			405,200
Share Issuance from/(to) Deferred Compensation Plan	(47)			$ 1,766	(47)	$ (1,766)
Share Issuance from/(to) Deferred Compensation Plan, Shares			(8,311)	153,771
Reallocation of Noncontrolling Interest	0				(297)					297
Preferred Share distributions	(6,900)								(6,900)
Distributions declared	(111,555)								(110,614)	(941)
Ending 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
Ending Balance, Shares at Dec. 31, 2016		4,000,000	175,140,760	899,457
Net income	121,859						120,850			1,009
Other comprehensive income	4,178							4,144		34
Issuance of partnership interest in consolidated real estate venture	85									85
Conversion of LP Units to Common Shares, Shares			2,858,991
Issuance of Common Shares of Beneficial Interest	51,225		$ 29		51,196
Redemption of Preferred Shares	$ (96,850)	$ (40)			(96,810)
Redemption of Preferred Shares, Shares		(4,000,000)
Repurchase and retirement of Common Shares of Beneficial Interest, Shares	0
Distributions from consolidated real estate ventures	$ (48)									(48)
Equity issuance costs	(499)				(499)
Bonus share issuance	110				110
Share-based compensation activity	6,697		$ 3		6,689		5
Share-based compensation activity, Shares			333,127	39,870
Share Issuance from/(to) Deferred Compensation Plan	$ (768)			$ (1,239)	(768)	1,239
Share Issuance from/(to) Deferred Compensation Plan, Shares			(52,971)	(44,591)
Share Choice Plan Issuance, Shares	0		(1,423)
Reallocation of Noncontrolling Interest	$ 0				(224)					224
Preferred Share distributions	(2,032)								(2,032)
Preferred Share redemption charges	(3,181)								(3,181)
Distributions declared	(117,613)								(116,636)	(977)
Ending Balance at Dec. 31, 2017	1,846,600	$ 0	$ 1,784	$ 12,445	3,218,564	(12,445)	660,174	2,399	(2,053,741)	17,420
Ending Balance, Shares at Dec. 31, 2017		0	178,285,236	894,736
Bonus share issuance, Shares			6,752
Net income	137,289						136,324			965
Other comprehensive income	2,669							2,630		39
Issuance of partnership interest in consolidated real estate venture	16									16
Conversion of LP Units to Common Shares, Shares			23,311
Issuance of Common Shares of Beneficial Interest	416				416
Repurchase and retirement of Common Shares of Beneficial Interest	$ (21,858)		$ (17)		(21,841)
Repurchase and retirement of Common Shares of Beneficial Interest, Shares	(1,729,278)		(1,729,278)
Distributions from consolidated real estate ventures	$ (94)									(94)
Redemption of LP Units	(7,043)									(7,043)
Share-based compensation activity	5,843		$ 2		5,826		15
Share-based compensation activity, Shares			196,151
Share Issuance from/(to) Deferred Compensation Plan	$ (111)		$ 1	$ 1,576	(112)	(1,576)
Share Issuance from/(to) Deferred Compensation Plan, Shares			99,189	82,384
Share Choice Plan Issuance, Shares	0		(1,285)
Reallocation of Noncontrolling Interest	$ 0				(2,003)					2,003
Preferred Share distributions	0
Distributions declared	(131,154)								(130,168)	(986)
Ending Balance at Dec. 31, 2018	$ 1,832,573	$ 0	$ 1,770	$ 14,021	$ 3,200,850	$ (14,021)	$ 796,513	$ 5,029	$ (2,183,909)	$ 12,320
Ending Balance, Shares at Dec. 31, 2018		0	176,873,324	977,120