Consolidated Balance Sheets (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Consolidated Balance Sheets
Accounts receivable, allowance for doubtful accounts (in dollars)	$ 3,796	$ 3,546
Preferred Shares of beneficial interest, aggregate liquidation preference (in dollars)	$ 216,333	$ 216,333
Preferred Shares of beneficial interest, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred Shares of beneficial interest, shares authorized	15,000,000	15,000,000
Preferred Shares of beneficial interest, shares issued	8,121,667	8,121,667
Preferred Shares of beneficial interest, shares outstanding	8,121,667	8,121,667
Common Shares of beneficial interest, par value (in dollars per share)	$ 0.01	$ 0.01
Common Shares of beneficial interest, shares authorized	125,000,000	125,000,000
Common Shares of beneficial interest, shares issued	72,037,627	72,011,324
Common Shares of beneficial interest, shares outstanding	72,037,627	72,011,324