Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Statement Of Financial Position [Abstract]
Intangible assets, accumulated amortization	$ 17.0	$ 19.5
Preferred shares of beneficial interest, par value	$ 1	$ 1
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	$ 1	$ 1
Common shares, shares authorized	95,000,000	95,000,000
Common shares, issued	13,783,442	13,771,287
Common shares, outstanding	13,783,442	13,771,287