Common Shares of Beneficial Interest (Schedule of ATM Equity Program) (Details) - Jun. 30, 2015 - ATM Equity Program [Member] - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	Total	Total
Class of Stock [Line Items]
Shares issued (in shares)	320	1,129
Weighted average price per share (in dollars per share)	$ 35.91	$ 36.18
Gross proceeds	$ 11,476	$ 40,836