Shareholders' Equity (Preferred shares of beneficial interest) (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Details Of Preferred Shares Of Beneficial Interest
Preferred stock, value	$ 891,214	$ 891,294
Preferred shares/units of beneficial interest: outstanding shares	36,795,640	36,798,580
Preferred shares/units of beneficial interest: authorized shares	110,000,000	110,000,000
Redeemable Preferred Stock
Details Of Preferred Shares Of Beneficial Interest
Preferred stock, liquidation preference per share	$ 25.00
Series A | Convertible Preferred Stock
Details Of Preferred Shares Of Beneficial Interest
Preferred stock, value	$ 991	$ 1,071
Preferred shares/units of beneficial interest: outstanding shares	15,640	18,580
Preferred stock, liquidation preference per share	$ 50.00
Preferred stock, dividend rate, per-dollar-amount	$ 3.25
Preferred shares/units of beneficial interest: authorized shares	15,640
Preferred stock dividend rate	6.50%
5.70% Series K | Redeemable Preferred Stock
Details Of Preferred Shares Of Beneficial Interest
Preferred stock, value	$ 290,971	$ 290,971
Preferred shares/units of beneficial interest: outstanding shares	12,000,000	12,000,000
Preferred stock, liquidation preference per share	$ 25.00
Preferred stock, dividend rate, per-dollar-amount	$ 1.425
Preferred shares/units of beneficial interest: authorized shares	12,000,000
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
Preferred shares/units of beneficial interest: outstanding shares	12,000,000	12,000,000
Preferred stock, liquidation preference per share	$ 25.00
Preferred stock, dividend rate, per-dollar-amount	$ 1.35
Preferred shares/units of beneficial interest: authorized shares	13,800,000
Preferred stock dividend rate	5.40%
Series M | Redeemable Preferred Stock
Details Of Preferred Shares Of Beneficial Interest
Preferred stock, value	$ 308,946	$ 308,946
Preferred shares/units of beneficial interest: outstanding shares	12,780,000	12,780,000
Preferred stock, liquidation preference per share	$ 25.00
Preferred stock, dividend rate, per-dollar-amount	$ 1.3125
Preferred shares/units of beneficial interest: authorized shares	13,800,000
Preferred stock dividend rate	5.25%