Shareholders' Equity (Preferred shares of beneficial interest) (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Sep. 01, 2016	Dec. 31, 2016	Dec. 31, 2015
Details Of Preferred Shares Of Beneficial Interest
Preferred Stock, Value		$ 1,038,055	$ 1,276,954
Preferred shares of beneficial interest: outstanding shares		42,824,829	52,676,629
Preferred shares of beneficial interest: authorized shares		110,000,000	110,000,000
Series A | Convertible Preferred Stock
Details Of Preferred Shares Of Beneficial Interest
Preferred Stock, Value		$ 1,264	$ 1,321
Preferred shares of beneficial interest: outstanding shares		24,829	26,629
Preferred Stock Liquidation Preference (in dollars per share)		$ 50
Preferred Stock, Dividend Rate, Per-Dollar-Amount		$ 3.25
Preferred shares of beneficial interest: authorized shares		83,977
Preferred Stock Dividend Rate		6.50%
Preferred Stock Redemption Price Per Share		$ 1.5934
Series G | Redeemable Preferred Stock
Details Of Preferred Shares Of Beneficial Interest
Preferred Stock, Value		$ 193,135	$ 193,135
Preferred shares of beneficial interest: outstanding shares		8,000,000	8,000,000
Preferred Stock Liquidation Preference (in dollars per share)		$ 25
Preferred Stock, Dividend Rate, Per-Dollar-Amount		$ 1.65625
Preferred shares of beneficial interest: authorized shares		8,000,000	8,000,000
Preferred Stock Dividend Rate		6.625%
Preferred Stock Redemption Price Per Share		$ 25
Series I | Redeemable Preferred Stock
Details Of Preferred Shares Of Beneficial Interest
Preferred Stock, Value		$ 262,379	$ 262,379
Preferred shares of beneficial interest: outstanding shares		10,800,000	10,800,000
Preferred Stock Liquidation Preference (in dollars per share)		$ 25
Preferred Stock, Dividend Rate, Per-Dollar-Amount		$ 1.65625
Preferred shares of beneficial interest: authorized shares		10,800,000	10,800,000
Preferred Stock Dividend Rate		6.625%
Preferred Stock Redemption Price Per Share		$ 25
Series J
Details Of Preferred Shares Of Beneficial Interest
Preferred Stock Dividend Rate	6.875%
Preferred Stock Redemption Price Per Share	$ 25
Series J | Redeemable Preferred Stock
Details Of Preferred Shares Of Beneficial Interest
Preferred Stock, Value		$ 0	$ 238,842
Preferred shares of beneficial interest: outstanding shares		0	9,850,000
Preferred shares of beneficial interest: authorized shares		9,850,000	9,850,000
Preferred Stock Dividend Rate		6.875%
Series K | Redeemable Preferred Stock
Details Of Preferred Shares Of Beneficial Interest
Preferred Stock, Value		$ 290,971	$ 290,971
Preferred shares of beneficial interest: outstanding shares		12,000,000	12,000,000
Preferred Stock Liquidation Preference (in dollars per share)		$ 25
Preferred Stock, Dividend Rate, Per-Dollar-Amount		$ 1.425
Preferred shares of beneficial interest: authorized shares		12,000,000	12,000,000
Preferred Stock Dividend Rate		5.70%
Preferred Stock Redemption Price Per Share		$ 25
Series L | Redeemable Preferred Stock
Details Of Preferred Shares Of Beneficial Interest
Preferred Stock, Value		$ 290,306	$ 290,306
Preferred shares of beneficial interest: outstanding shares		12,000,000	12,000,000
Preferred Stock Liquidation Preference (in dollars per share)		$ 25
Preferred Stock, Dividend Rate, Per-Dollar-Amount		$ 1.35
Preferred shares of beneficial interest: authorized shares		12,000,000	12,000,000
Preferred Stock Dividend Rate		5.40%
Preferred Stock Redemption Price Per Share		$ 25