Shareholders' Equity (Preferred shares of beneficial interest) (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Details Of Preferred Shares Of Beneficial Interest
Preferred stock, value	$ 891,988	$ 891,988	$ 1,038,055
Preferred shares of beneficial interest: outstanding shares	36,799,573	36,799,573	42,824,829
Preferred shares of beneficial interest: authorized shares	110,000,000	110,000,000	110,000,000
Redeemable shares, conversion ratio		195.31%
Cumulative redeemable preferred shares called for redemption	$ 455,514	$ 455,514	$ 0
Redeemable Preferred Stock
Details Of Preferred Shares Of Beneficial Interest
Preferred stock, liquidation preference per share	$ 25.00	$ 25.00
Series A | Convertible Preferred Stock
Details Of Preferred Shares Of Beneficial Interest
Preferred stock, value	$ 1,102	$ 1,102	$ 1,264
Preferred shares of beneficial interest: outstanding shares	19,573	19,573	24,829
Preferred stock, liquidation preference per share	$ 50.00	$ 50.00
Preferred stock, dividend rate, per-dollar-amount		$ 3.25
Preferred shares of beneficial interest: authorized shares	83,977,000	83,977,000
Preferred stock dividend rate		6.50%
Series G | Redeemable Preferred Stock
Details Of Preferred Shares Of Beneficial Interest
Preferred stock, value	$ 0	$ 0	$ 193,135
Preferred shares of beneficial interest: outstanding shares	0	0	8,000,000
Preferred stock, liquidation preference per share	$ 25.00	$ 25.00
Preferred stock, dividend rate, per-dollar-amount		$ 1.65625
Preferred shares of beneficial interest: authorized shares	8,000,000,000	8,000,000,000
Preferred stock dividend rate		6.625%
Series I | Redeemable Preferred Stock
Details Of Preferred Shares Of Beneficial Interest
Preferred stock, value	$ 0	$ 0	$ 262,379
Preferred shares of beneficial interest: outstanding shares	0	0	10,800,000
Preferred stock, liquidation preference per share	$ 25.00	$ 25.00
Preferred stock, dividend rate, per-dollar-amount		$ 1.65625
Preferred shares of beneficial interest: authorized shares	10,800,000,000	10,800,000,000
Preferred stock dividend rate		6.625%
Series K | Redeemable Preferred Stock
Details Of Preferred Shares Of Beneficial Interest
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 per share	$ 25.00	$ 25.00
Preferred stock, dividend rate, per-dollar-amount		$ 1.425
Preferred shares of beneficial interest: authorized shares	12,000,000,000	12,000,000,000
Preferred stock dividend rate		5.70%
Series L | Redeemable Preferred Stock
Details Of Preferred Shares Of Beneficial Interest
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 per share	$ 25.00	$ 25.00
Preferred stock, dividend rate, per-dollar-amount		$ 1.35
Preferred shares of beneficial interest: authorized shares	12,000,000,000	12,000,000,000
Preferred stock dividend rate		5.40%
Series M | Redeemable Preferred Stock
Details Of Preferred Shares Of Beneficial Interest
Preferred stock, value	$ 309,609	$ 309,609	$ 0
Preferred shares of beneficial interest: outstanding shares	12,780,000	12,780,000	0
Preferred stock, liquidation preference per share	$ 25.00	$ 25.00
Preferred stock, dividend rate, per-dollar-amount		$ 1.31
Preferred shares of beneficial interest: authorized shares	12,780,000,000	12,780,000,000
Preferred stock, redemption price per share	$ 25.00	$ 25.00
Preferred stock dividend rate	5.25%	5.25%