Common Shares Of Beneficial Interest (Details) (USD $)	1 Months Ended	12 Months Ended			3 Months Ended		2 Months Ended
	Dec. 31, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014	Dec. 31, 2013	Feb. 19, 2015
Class of Stock [Line Items]
Common shares of beneficial interest, dividend per share	$ 0.25	$ 1.55	$ 1.22	$ 1.16
Quarterly Rate Per Share [Member]
Class of Stock [Line Items]
Common shares of beneficial interest, dividend per share					$ 0.325	$ 0.305
Quarterly Rate Per Share [Member] | Subsequent Event [Member]
Class of Stock [Line Items]
Common shares of beneficial interest, dividend per share							$ 0.345