Common Shares of Beneficial Interest (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2020	Sep. 30, 2020	Dec. 31, 2019	Nov. 04, 2020
Class of Stock [Line Items]
Value of shares approved to be repurchased	$ 200,000	$ 200,000	$ 200,000
Common Shares of Beneficial Interest; par value (dollars per share)	$ 0.03	$ 0.03	$ 0.03	$ 0.03
Common dividends declared (in dollars per share)	$ 0.18	$ 0.18
Dividends declared and payable	$ 23,066	$ 23,066	$ 23,066
Common Shares of Beneficial Interest
Class of Stock [Line Items]
Stock repurchase program, remaining authorized repurchase amount	$ 163,300	$ 163,300	$ 163,300
Stock Repurchased During Period, Shares			800,000	0
Common Shares of Beneficial Interest; par value (dollars per share)	$ 21.47	$ 21.47	$ 21.47
Subsequent Event [Member]
Class of Stock [Line Items]
Stock repurchase program, remaining authorized repurchase amount					$ 163,300