Common Shares of Beneficial Interest (Narrative) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	6 Months Ended
	Jun. 30, 2018	Aug. 01, 2018	Dec. 31, 2017
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
Stock repurchase program, remaining authorized repurchase amount	$ 181.5
Common Stock [Member]
Class of Stock [Line Items]
Stock repurchased during period (in shares)	0.7
Common shares of beneficial interest; par value (dollars per share)	$ 27.10
Subsequent Event [Member]
Class of Stock [Line Items]
Stock repurchase program, remaining authorized repurchase amount		$ 181.5