Preferred Shares Of Beneficial Interest (Details) (USD $)	12 Months Ended			0 Months Ended	12 Months Ended			1 Months Ended		12 Months Ended		0 Months Ended	1 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Jun. 06, 2008 6.5% Series F Preferred Shares [Member]	Dec. 31, 2012 6.5% Series F Preferred Shares [Member]	Dec. 31, 2011 6.5% Series F Preferred Shares [Member]	Jan. 30, 2007 6.5% Series F Preferred Shares [Member]	Nov. 30, 2012 6.95% Series E Preferred Shares [Member]	Jul. 31, 2004 6.95% Series E Preferred Shares [Member]	Dec. 31, 2012 6.95% Series E Preferred Shares [Member]	Dec. 31, 2011 6.95% Series E Preferred Shares [Member]	Feb. 14, 2013 6.75% Series D Preferred Shares [Member]	Apr. 30, 2003 6.75% Series D Preferred Shares [Member]	Dec. 31, 2012 6.75% Series D Preferred Shares [Member]	Dec. 31, 2011 6.75% Series D Preferred Shares [Member]
Class of Stock [Line Items]
Value of depositary shares issued				$ 150,000,000	$ 4,000	$ 4,000	$ 200,000,000		$ 72,500,000	$ 0	$ 1,000		$ 75,000,000	$ 3,000	$ 3,000
Depositary shares, conversion basis					0.01				0.01				0.0333
Redemption price per depositary share					$ 25			$ 25					$ 25
Annual dividend rate of preferred shares					6.50%	6.50%			6.95%	6.95%	6.95%	6.75%	6.75%	6.75%	6.75%
Liquidation value per preferred share					$ 2,500				$ 2,500				$ 750
Effective dividend yield on preferred shares				8.25%
Redemption value paid								72,500,000				75,000,000
Redemption costs of preferred shares	$ 2,500,000	$ 0	$ 0					$ 2,500,000
Redemption date								Nov. 21, 2012				Mar. 18, 2013