Common Shares Of Beneficial Interest (Details) - USD ($)	1 Months Ended	3 Months Ended									12 Months Ended
	Dec. 31, 2014	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	May. 31, 2015	Feb. 28, 2015	Apr. 18, 2013
Class of Stock [Line Items]
Stock repurchase program, number of shares authorized to be repurchased (in shares)			200,000,000								200,000,000
Common shares of beneficial interest, dividend per share (in dollars per share)	$ 0.25										$ 1.38	$ 1.55	$ 1.22
ATM Equity Program [Member]
Class of Stock [Line Items]
Equity issuances amount available for issuance			$ 159,200,000								$ 159,200,000
Shares sold (in shares)											1,129,000
Weighted average price per share (in dollars per share)											$ 36.18
Gross proceeds											$ 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)			$ 0.345	$ 345.000	$ 345.000	$ 345.000	$ 0.325	$ 325.000	$ 325.000	$ 325.000
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
Maximum [Member] | ATM Equity Program [Member]
Class of Stock [Line Items]
Equity issuances amount available for issuance															$ 200,000,000
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)		$ 0.365