CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Statement Of Financial Position [Abstract]
Intangible assets, accumulated amortization	$ 28.7	$ 27.1
Preferred shares of beneficial interest, par value	$ 0.01	$ 0.01
Preferred shares of beneficial interest, shares authorized	5,000,000	5,000,000
Preferred shares of beneficial interest, issued	0	0
Preferred shares of beneficial interest, outstanding	0	0
Common shares, par value	$ 0.01	$ 0.01
Common shares, shares authorized	95,000,000	95,000,000
Common shares, issued	13,735,369	13,599,055
Common shares, outstanding	13,735,369	13,599,055