Preferred Shares Of Beneficial Interest (Narrative) (Details) (USD $)	3 Months Ended		1 Months Ended	3 Months Ended	12 Months Ended	0 Months Ended	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Apr. 30, 2003 Series D Preferred Stock [Member]	Mar. 31, 2013 Series D Preferred Stock [Member]	Dec. 31, 2012 Series D Preferred Stock [Member]	May 06, 2013 6.5% Series F Preferred Shares [Member]	Mar. 31, 2013 6.5% Series F Preferred Shares [Member]	Dec. 31, 2012 6.5% Series F Preferred Shares [Member]
Class of Stock [Line Items]
Preferred Shares of Beneficial Interest; value			$ 75,000,000	$ 0	$ 3,000		$ 4,000	$ 4,000
Preferred Shares of Beneficial Interest; depositary shares, conversion basis			0.0333
Preferred Shares of Beneficial Interest; redemption date				Mar. 18, 2013		Jun. 05, 2013
Preferred Shares of Beneficial Interest; redemption price per depositary share				$ 25
Preferred Shares of Beneficial Interest; redemption value paid				75,000,000		200,000,000
Redemption costs of preferred shares	$ 2,242,000	$ 0		$ 2,200,000
Preferred Shares of Beneficial Interest; dividend percentage			6.75%	6.75%	6.75%	6.50%	6.50%	6.50%
Preferred Shares of Beneficial Interest; liquidation value per share			$ 750