Preferred Shares Of Beneficial Interest (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	0 Months Ended	9 Months Ended	12 Months Ended		1 Months Ended	9 Months Ended	12 Months Ended	1 Months Ended	9 Months Ended	12 Months Ended
	Jun. 06, 2008 6.5% Series F Preferred Shares [Member]	Sep. 30, 2011 6.5% Series F Preferred Shares [Member]	Dec. 31, 2010 6.5% Series F Preferred Shares [Member]	Jan. 30, 2007 6.5% Series F Preferred Shares [Member]	Jul. 31, 2004 6.95% Series E Preferred Shares [Member]	Sep. 30, 2011 6.95% Series E Preferred Shares [Member]	Dec. 31, 2010 6.95% Series E Preferred Shares [Member]	Apr. 30, 2003 6.75% Series D Preferred Shares [Member]	Sep. 30, 2011 6.75% Series D Preferred Shares [Member]	Dec. 31, 2010 6.75% Series D Preferred Shares [Member]
Preferred Shares Of Beneficial Interest [Line Items]
Value of depositary shares issued	$ 150.0			$ 200.0	$ 72.5			$ 75.0
Depositary shares, redemption date		Jan. 30, 2012
Redemption price per 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%
Liquidation value per share		$ 2,500			$ 2,500			$ 750
Effective dividend yield on preferred shares	8.25%
Depositary shares, conversion basis		0.01			0.01			0.033333333333