Common Shares Of Beneficial Interest (Details) - USD ($)	1 Months Ended	2 Months Ended	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Feb. 28, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Aug. 31, 2016	Mar. 30, 2016	Oct. 31, 2015	May 31, 2015	Feb. 28, 2015
Class of Stock [Line Items]
Stock repurchase program, number of shares authorized to be repurchased (in shares)																200,000,000
Common shares of beneficial interest, dividend per share (in dollars per share)	$ 0.25										$ 1.46	$ 1.38	$ 1.55
ATM Equity Program [Member]
Class of Stock [Line Items]
Equity issuances amount available for issuance														$ 34,100,000
Shares sold (in shares)											3,465,000	1,129,000
Weighted average price per share (in dollars per share)											$ 38.35	$ 36.18
Gross proceeds											$ 132,884,000	$ 40,836,000
Quarterly Rate Per Share [Member]
Class of Stock [Line Items]
Common shares of beneficial interest, dividend per share (in dollars per share)			$ 0.365	$ 0.365	$ 0.365	$ 0.365	$ 0.345	$ 0.345	$ 0.345	$ 0.345
Unsecured Revolving Credit Facility [Member]
Class of Stock [Line Items]
Maximum borrowing capacity under credit facility							$ 500,000,000			$ 500,000,000		$ 500,000,000		500,000,000	$ 500,000,000		$ 500,000,000	$ 500,000,000
Maximum [Member] | ATM Equity Program [Member]
Class of Stock [Line Items]
Equity issuances amount available for issuance			$ 0								$ 0			$ 250,000,000				$ 200,000,000
Subsequent Event [Member] | ATM Equity Program [Member]
Class of Stock [Line Items]
Equity issuances amount available for issuance		$ 242,200,000
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)		$ 0.385