CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
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)	121,922,120	121,922,120	121,572,155
Common shares of beneficial interest, shares outstanding (in shares)	121,922,120	121,922,120	121,572,155
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