Common Shares Of Beneficial Interest (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	2 Months Ended	3 Months Ended								12 Months Ended
	Feb. 28, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Stock repurchase program, authorized amount		$ 200.0								$ 200.0
Common shares of beneficial interest, dividend per share (in dollars per share)										$ 2.980	$ 2.29	$ 1.46
Common stock, dividends, cash paid (in dollars per share)										1.40	0.75
Common shares of beneficial interest; par value		$ 0.03				$ 0.03				$ 0.03	$ 0.03
Stock Repurchase Program, Remaining Authorized Repurchase Amount		$ 181.5								$ 181.5
Subsequent Event [Member]
Class of Stock [Line Items]
Stock Repurchase Program, Remaining Authorized Repurchase Amount	$ 181.5
Quarterly Rate Per Share [Member]
Class of Stock [Line Items]
Common shares of beneficial interest, dividend per share (in dollars per share)		$ 0.395	$ 0.395	$ 0.395	$ 0.395	$ 0.385	$ 0.385	$ 0.385	$ 0.385
Quarterly Rate Per Share [Member] | Subsequent Event [Member]
Class of Stock [Line Items]
Common shares of beneficial interest, dividend per share (in dollars per share)	$ 0.395
Common Shares of Beneficial Interest
Class of Stock [Line Items]
Stock repurchased during period, shares										0.7
Common shares of beneficial interest; par value		$ 27.10								$ 27.10