Common Shares of Beneficial Interest (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2020	Dec. 31, 2019
Class of Stock [Line Items]
Value of shares approved to be repurchased	$ 200.0
Common Shares of Beneficial Interest; par value (dollars per share)	$ 0.03	$ 0.03
Common Shares of Beneficial Interest
Class of Stock [Line Items]
Stock repurchase program, remaining authorized repurchase amount	$ 163.3
Stock Repurchased During Period, Shares	800,000	0
Common Shares of Beneficial Interest; par value (dollars per share)	$ 21.47