Preferred Shares Of Beneficial Interest (Details) (USD $)	12 Months Ended			3 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jun. 30, 2013	Jun. 06, 2008	Jan. 30, 2007
Class of Stock [Line Items]
Preferred shares of beneficial interest, value	$ 2,000	$ 2,000
Redemption costs of preferred shares	0	17,944,000	2,500,000
6.5% Series F Preferred Shares [Member]
Class of Stock [Line Items]
Preferred shares of beneficial interest, value					150,000,000	200,000,000
Preferred shares of beneficial interest, depositary shares, conversion basis	0.01
Preferred shares of beneficial interest, redemption price per depositary share	$ 25
Preferred shares of beneficial interest, dividend percentage	6.50%	6.50%
Preferred shares of beneficial interest, liquidation value per share	$ 2,500
Preferred shares of beneficial interest, effective dividend yield percentage	8.25%
Preferred Shares of beneficial interest, redemption value paid		200,000,000
Preferred shares of beneficial interest, liquidation preference	150,000,000	150,000,000
Preferred shares of beneficial interest, redemption date		Jun. 05, 2013
Redemption costs of preferred shares		15,700,000		15,700,000
6.5% Series F Preferred Shares [Member] | Preferred Shares Issued At Discount [Member]
Class of Stock [Line Items]
Preferred shares of beneficial interest, effective dividend yield percentage	8.25%
Preferred shares of beneficial interest, liquidation preference	$ 64,300,000