Consolidated Balance Sheets (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Statement of Financial Position [Abstract]
Accounts receivable, allowance for doubtful accounts	$ 4,754	$ 3,546
Preferred Shares of beneficial interest, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred Shares of beneficial interest, shares authorized	25,000,000	15,000,000
Preferred Shares of beneficial interest, shares issued	12,821,667	8,121,667
Preferred Shares of beneficial interest, shares outstanding	12,821,667	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	72,157,635	72,011,324
Common Shares of beneficial interest, shares outstanding	72,157,635	72,011,324