Condensed Consolidated Balance Sheets (Parenthetical) (USD $) In Thousands, except Per Share data, unless otherwise specified			3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 6.75% Series D Preferred Shares [Member]	Dec. 31, 2012 6.75% Series D Preferred Shares [Member]	Mar. 31, 2013 6.5% Series F Preferred Shares [Member]	Dec. 31, 2012 6.5% Series F Preferred Shares [Member]
ASSETS
Allowance for doubtful accounts	$ 11,904	$ 12,127
Preferred Shares of Beneficial Interest - par value, $.03 per share; shares authorized: 10,000
Preferred Shares of Beneficial Interest; par value per share	$ 0.03	$ 0.03
Preferred Shares of Beneficial Interest; shares authorized	10,000	10,000
Preferred Shares of Beneficial Interest; shares issued			100	100	140	140
Preferred Shares of Beneficial Interest; shares outstanding			0	100	140	140
Preferred Shares of Beneficial Interest; liquidation preference			$ 0	$ 75,000	$ 350,000	$ 350,000
Preferred Shares of Beneficial Interest; dividend percentage			6.75%	6.75%	6.50%	6.50%
Common Shares of Beneficial Interest—par value, $.03 per share; shares authorized: 275,000; shares issued and outstanding: 121,758 in 2013 and 121,505 in 2012
Common Shares of Beneficial Interest; par value per share	$ 0.03	$ 0.03
Common Shares of Beneficial Interest; shares authorized	275,000	275,000
Common Shares of Beneficial Interest; shares issued	121,758	121,505
Common Shares of Beneficial Interest; shares outstanding	121,758	121,505