CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) $ in Thousands	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2017 USD ($) $ / shares shares	Jun. 30, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares
Rents receivable, allowance for doubtful accounts | $	$ 4,352	$ 4,352	$ 5,105
Common shares of beneficial interest, par value (in dollars per share) | $ / shares	$ 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)	124,089,443	124,089,443	123,994,465
Common shares of beneficial interest, shares outstanding (in shares)	124,089,443	124,089,443	123,994,465
Series D
Preferred shares of beneficial interest, par value (in dollars per share) | $ / shares	$ 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
Common shares of beneficial interest, shares issued (in shares)	4,915,196	4,915,196	4,915,196