Consolidated Balance Sheets (Parenthetical) (USD $) In Thousands, except Per Share data, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Statement of Financial Position [Abstract]
Rents and other receivables, allowance for doubtful accounts	$ 9,585	$ 10,958
Preferred Stock, Par or Stated Value Per Share (in dollars per share)	$ 0.01	$ 0.01
Preferred Stock, Shares Authorized (in shares)	10,000	10,000
Preferred Stock, Shares Issued (in shares)	0	0
Preferred Stock, Shares Outstanding (in shares)	0	0
Shares of beneficial interest, par value (in dollars per share)	$ 0.01	$ 0.01
Shares of beneficial interest, shares authorized (in shares)	100,000	100,000
Shares of beneficial interest, shares issued (in shares)	66,500	66,437
Shares of beneficial interest, shares outstanding (in shares)	66,500	66,437