Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Rents and other receivables, allowance for doubtful accounts	$ 2,494	$ 2,377
Preferred shares, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred shares, shares authorized (in shares)	10,000,000	10,000,000
Preferred shares, shares issued (in shares)	0	0
Preferred shares, 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)	100,000,000	100,000,000
Shares of beneficial interest, shares issued (in shares)	78,464,000	74,606,000
Shares of beneficial interest, shares outstanding (in shares)	78,464,000	74,606,000