Common Shares Of Beneficial Interest (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	2 Months Ended	3 Months Ended								12 Months Ended
	Feb. 28, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Stock repurchase program, authorized amount		$ 200,000								$ 200,000
Common shares of beneficial interest, dividend per share (in dollars per share)										$ 2.290	$ 1.460	$ 1.38
Common stock, dividends, cash paid (in dollars per share)										$ 0.75
ATM Equity Program [Member]
Class of Stock [Line Items]
Equity issuances amount available for issuance		$ 0								$ 0
Shares sold (in shares)										0	3,465
Weighted average price per share (in dollars per share)											$ 38.35
Gross proceeds											$ 132,884
Quarterly Rate Per Share [Member]
Class of Stock [Line Items]
Common shares of beneficial interest, dividend per share (in dollars per share)		$ 0.385	$ 0.385	$ 0.385	$ 0.385	$ 0.365	$ 0.365	$ 0.365	$ 0.365
Maximum [Member] | ATM Equity Program [Member]
Class of Stock [Line Items]
Equity issuances amount available for issuance		$ 250,000								$ 250,000
Subsequent Event [Member] | Quarterly Rate Per Share [Member]
Class of Stock [Line Items]
Common shares of beneficial interest, dividend per share (in dollars per share)	$ 0.395