Common Shares of Beneficial Interest (Narrative) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	1 Months Ended
	May 02, 2018	Mar. 31, 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		$ 191.9
Common Stock [Member]
Class of Stock [Line Items]
Stock repurchased during period (in shares)		0.3
Common shares of beneficial interest; par value (dollars per share)		$ 27.37
Subsequent Event [Member]
Class of Stock [Line Items]
Stock repurchased during period (in shares)	0.3
Stock repurchase program, remaining authorized repurchase amount	$ 185.0