Consolidated Statements Of Equity (USD $)	Total	Preferred Shares Of Beneficial Interest [Member]	Common Shares Of Beneficial Interest [Member]	Additional Paid-In Capital [Member]	Net Income Less Than Accumulated Dividends [Member]	Accumulated Other Comprehensive Loss [Member]	Noncontrolling Interests [Member]
Balance at Dec. 31, 2011	$ 1,823,582,000	$ 8,000	$ 3,641,000	$ 1,983,978,000	$ (304,504,000)	$ (27,743,000)	$ 168,202,000
Increase (Decrease) in Equity [Roll Forward]
Net income	152,421,000				146,640,000		5,781,000
Redemption of preferred shares	(72,500,000)	(1,000)		(69,999,000)	(2,500,000)
Shares issued under benefit plans	16,590,000		22,000	16,568,000
Dividends paid - common shares	(140,686,000)				(140,686,000)
Dividends paid - preferred shares	(32,516,000)				(32,516,000)
Distributions to noncontrolling interests	(12,770,000)						(12,770,000)
Contributions from noncontrolling interests	2,123,000						2,123,000
Other comprehensive income (loss)	3,000,000					3,000,000
Other, net	911,000			3,636,000	(2,414,000)		(311,000)
Balance at Dec. 31, 2012	1,740,155,000	7,000	3,663,000	1,934,183,000	(335,980,000)	(24,743,000)	163,025,000
Increase (Decrease) in Equity [Roll Forward]
Net income	265,156,000				220,262,000		44,894,000
Redemption of preferred shares	(275,000,000)	(5,000)		(257,051,000)	(17,944,000)
Shares issued under benefit plans	13,608,000		20,000	13,588,000
Dividends paid - common shares	(148,702,000)				(148,702,000)
Dividends paid - preferred shares	(17,198,000)				(17,198,000)
Distributions to noncontrolling interests	(20,151,000)						(20,151,000)
Contributions from noncontrolling interests	106,613,000						106,613,000
Acquisition of noncontrolling interests	0			(16,177,000)			16,177,000
Other comprehensive income (loss)	20,541,000					20,541,000
Other, net	2,956,000			4,686,000	(975,000)		(755,000)
Balance at Dec. 31, 2013	1,687,978,000	2,000	3,683,000	1,679,229,000	(300,537,000)	(4,202,000)	309,803,000
Increase (Decrease) in Equity [Roll Forward]
Net income	307,579,000				288,008,000		19,571,000
Shares issued under benefit plans	15,898,000		17,000	15,881,000
Dividends paid - common shares	(189,591,000)				(189,591,000)
Dividends paid - preferred shares	(9,752,000)				(9,752,000)
Distributions to noncontrolling interests	(21,055,000)						(21,055,000)
Contributions from noncontrolling interests	980,000						980,000
Acquisition of noncontrolling interests	0			11,015,000			(11,015,000)
Disposition of noncontrolling interests	(144,263,000)						(144,263,000)
Other comprehensive income (loss)	(8,234,000)					(8,234,000)
Other, net	(597,000)			755,000	(1,088,000)		(264,000)
Balance at Dec. 31, 2014	$ 1,638,943,000	$ 2,000	$ 3,700,000	$ 1,706,880,000	$ (212,960,000)	$ (12,436,000)	$ 153,757,000