CONDENSED CONSOLIDATED BALANCE SHEETS (unaudited) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Rents receivable, allowance for doubtful accounts	$ 4,431	$ 4,431	$ 7,715
Common shares of beneficial interest, par value (in dollars per share)	$ 0.01	$ 0.01	$ 0.01
Common shares of beneficial interest, shares authorized (in shares)	350,000,000	350,000,000	350,000,000
Common shares of beneficial interest, shares issued (in shares)	125,533,376	125,533,376	126,349,914
Common shares of beneficial interest, shares outstanding (in shares)	125,533,376	125,533,376	126,349,914
Series D
Preferred shares of beneficial interest, par value (in dollars per share)	$ 0.01	$ 0.01	$ 0.01
Preferred shares of beneficial interest, shares authorized (in shares)	50,000,000	50,000,000	50,000,000
Preferred shares, dividend yield	6.50%	6.50%	6.50%
Preferred shares of beneficial interest, shares issued (in shares)	4,915,196	4,915,196	4,915,196
Preferred shares, of beneficial interest, shares outstanding (in shares)	4,915,196	4,915,196	4,915,196
Preferred shares, aggregate liquidation preference	$ 122,880	$ 122,880	$ 122,880
Series E
Preferred shares of beneficial interest, par value (in dollars per share)	$ 0	$ 0	$ 0.01
Preferred shares of beneficial interest, shares authorized (in shares)	0	0	50,000,000
Preferred shares, dividend yield		0.00%	7.25%
Preferred shares of beneficial interest, shares issued (in shares)	0	0	11,000,000
Preferred shares, of beneficial interest, shares outstanding (in shares)	0	0	11,000,000
Preferred shares, aggregate liquidation preference	$ 0	$ 0	$ 275,000