Common Shares of Beneficial Interest (Schedule of ATM Equity Program) (Details) - ATM Equity Program [Member] - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Class of Stock [Line Items]
Shares sold (in shares)	2,792	320	3,277	1,129
Weighted average price per share (in dollars per share)	$ 38.32	$ 35.91	$ 38.16	$ 36.18
Gross proceeds	$ 106,992	$ 11,476	$ 125,058	$ 40,836