Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Common Stock Number Of Shares Par Value And Other Disclosures [Abstract]
Common shares of beneficial interest: outstanding shares	188,576,853				188,576,853	187,887,498
Dividends paid on common shares					$ 474,751	$ 547,831	$ 545,913
Dividends per common share	$ 0.63	$ 0.63	$ 0.63	$ 0.63
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Preferred shares of beneficial interest: issued shares	52,676,629				52,676,629	52,678,939
Details Of Preferred Shares Of Beneficial Interest [Abstract]
Preferred Stock, Value	$ 1,276,954				$ 1,276,954	$ 1,277,026
Preferred shares of beneficial interest: outstanding shares	52,676,629				52,676,629	52,678,939
Preferred shares of beneficial interest: authorized shares	110,000,000				110,000,000	110,000,000
Series A Preferred Stock [Member] | Convertible Preferred Stock [Member]
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Preferred Stock Dividend Rate Percentage					6.50%
Details Of Preferred Shares Of Beneficial Interest [Abstract]
Preferred Stock, Value	$ 1,321				$ 1,321	$ 1,393
Preferred shares of beneficial interest: outstanding shares	26,629				26,629	28,939
Preferred Stock Liquidation Preference (in dollars per share)	$ 50.00				$ 50.00
Preferred Stock, Dividend Rate, Per-Dollar-Amount					$ 3.25
Preferred shares of beneficial interest: authorized shares	83,977				83,977	83,977
Conversion Rate of Preferred Stock to Common Stock, Shares	1.5934				1.5934	1.4334
Series G Preferred Stock [Member] | Redeemable Preferred Stock [Member]
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Preferred Stock Dividend Rate Percentage					6.625%
Preferred Stock Redemption Price Per Share	$ 25.00				$ 25.00
Details Of Preferred Shares Of Beneficial Interest [Abstract]
Preferred Stock, Value	$ 193,135				$ 193,135	$ 193,135
Preferred shares of beneficial interest: outstanding shares	8,000,000				8,000,000	8,000,000
Preferred Stock Liquidation Preference (in dollars per share)	$ 25.00				$ 25.00
Preferred Stock, Dividend Rate, Per-Dollar-Amount					$ 1.65625
Preferred shares of beneficial interest: authorized shares	8,000,000				8,000,000	8,000,000
Series I Preferred Stock [Member] | Redeemable Preferred Stock [Member]
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Preferred Stock Dividend Rate Percentage					6.625%
Preferred Stock Redemption Price Per Share	$ 25.00				$ 25.00
Details Of Preferred Shares Of Beneficial Interest [Abstract]
Preferred Stock, Value	$ 262,379				$ 262,379	$ 262,379
Preferred shares of beneficial interest: outstanding shares	10,800,000				10,800,000	10,800,000
Preferred Stock Liquidation Preference (in dollars per share)	$ 25.00				$ 25.00
Preferred Stock, Dividend Rate, Per-Dollar-Amount					$ 1.65625
Preferred shares of beneficial interest: authorized shares	10,800,000				10,800,000	10,800,000
Series J Preferred Stock [Member] | Redeemable Preferred Stock [Member]
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Preferred Stock Dividend Rate Percentage					6.875%
Preferred Stock Redemption Price Per Share	$ 25.00				$ 25.00
Details Of Preferred Shares Of Beneficial Interest [Abstract]
Preferred Stock, Value	$ 238,842				$ 238,842	$ 238,842
Preferred shares of beneficial interest: outstanding shares	9,850,000				9,850,000	9,850,000
Preferred Stock Liquidation Preference (in dollars per share)	$ 25.00				$ 25.00
Preferred Stock, Dividend Rate, Per-Dollar-Amount					$ 1.71875
Preferred shares of beneficial interest: authorized shares	9,850,000				9,850,000	9,850,000
Series K Preferred Stock [Member] | Redeemable Preferred Stock [Member]
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Preferred Stock Dividend Rate Percentage					5.70%
Preferred Stock Redemption Price Per Share	$ 25.00				$ 25.00
Details Of Preferred Shares Of Beneficial Interest [Abstract]
Preferred Stock, Value	$ 290,971				$ 290,971	$ 290,971
Preferred shares of beneficial interest: outstanding shares	12,000,000				12,000,000	12,000,000
Preferred Stock Liquidation Preference (in dollars per share)	$ 25.00				$ 25.00
Preferred Stock, Dividend Rate, Per-Dollar-Amount					$ 1.425
Preferred shares of beneficial interest: authorized shares	12,000,000				12,000,000	12,000,000
Series L Preferred Stock [Member] | Redeemable Preferred Stock [Member]
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Preferred Stock Dividend Rate Percentage					5.40%
Preferred Stock Redemption Price Per Share	$ 25.00				$ 25.00
Details Of Preferred Shares Of Beneficial Interest [Abstract]
Preferred Stock, Value	$ 290,306				$ 290,306	$ 290,306
Preferred shares of beneficial interest: outstanding shares	12,000,000				12,000,000	12,000,000
Preferred Stock Liquidation Preference (in dollars per share)	$ 25.00				$ 25.00
Preferred Stock, Dividend Rate, Per-Dollar-Amount					$ 1.35
Preferred shares of beneficial interest: authorized shares	12,000,000				12,000,000	12,000,000