Preferred Shares Of Beneficial Interest (Narrative) (Details) (USD $)	3 Months Ended		6 Months Ended		1 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended			6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Apr. 30, 2003 6.75% Series D Preferred Shares [Member]	Jun. 30, 2013 6.75% Series D Preferred Shares [Member]	Dec. 31, 2012 6.75% Series D Preferred Shares [Member]	Jun. 30, 2013 6.5% Series F Preferred Shares [Member]	Dec. 31, 2012 6.5% Series F Preferred Shares [Member]	Jun. 06, 2008 6.5% Series F Preferred Shares [Member]	Jan. 30, 2007 6.5% Series F Preferred Shares [Member]	Jun. 30, 2013 6.5% Series F Preferred Shares [Member] Preferred Shares Issued At Discount [Member]
Class of Stock [Line Items]
Preferred Shares of Beneficial Interest - value					$ 75,000,000	$ 0	$ 3,000	$ 2,000	$ 4,000	$ 150,000,000	$ 200,000,000
Preferred Shares of Beneficial Interest; depositary shares, conversion basis					0.0333			0.01
Preferred Shares of Beneficial Interest; redemption price per depositary share						$ 25		$ 25
Preferred Shares of Beneficial Interest; redemption date						Mar. 18, 2013		Jun. 05, 2013
Preferred Shares of Beneficial Interest; redemption value paid						75,000,000		200,000,000
Redemption costs of preferred shares	15,702,000	0	17,944,000	0		2,200,000		15,700,000
Preferred Shares of Beneficial Interest; liquidation preference						$ 0	$ 75,000,000	$ 150,000,000	$ 350,000,000			$ 64,300,000
Preferred Shares of Beneficial Interest; dividend percentage					6.75%	6.75%	6.75%	6.50%	6.50%
Effective dividend yield on preferred shares												8.25%
Preferred Shares of Beneficial Interest; liquidation value per share					$ 750			$ 2,500