Common Shares of Beneficial Interest (Schedule of ATM Equity Program) (Details) - ATM Equity Program [Member] - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2015
Class of Stock [Line Items]
Shares sold (in shares)	0	1,129,000
Weighted average price per share (in dollars per share)		$ 36.18
Gross proceeds		$ 40,836