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. 26, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Class of Stock [Line Items]
Value of shares approved to be repurchased		$ 200.0												$ 200.0
Stock Repurchased During Period, Shares														1.7	0.0
Average price of share repurchased (in dollars per share)														$ 19.09
Common shares of beneficial interest, dividend per share (in dollars per share)														1.30	$ 1.58	$ 2.98
Common dividend paid per share (in dollars per share)														1.30	1.58	2.98
Common shares of beneficial interest; par value		$ 0.03				$ 0.03								$ 0.03	0.03
Stock repurchase program, remaining authorized repurchase amount	$ 149.4	$ 149.4												$ 149.4
Special Dividend [Member]
Class of Stock [Line Items]
Common dividend paid per share (in dollars per share)														$ 0.36	$ 0	$ 1.40
Quarterly Rate Per Share [Member]
Class of Stock [Line Items]
Common shares of beneficial interest, dividend per share (in dollars per share)		$ 0.18	$ 0.18	$ 0.18	$ 0.395	$ 0.395	$ 0.395	$ 0.395	$ 0.395	$ 0.395	$ 0.395	$ 0.395	$ 0.395
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)	$ 0.30