Common Shares of Beneficial Interest (Schedule of ATM Equity Program) (Details) - ATM Equity Program [Member] - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Class of Stock [Line Items]
Shares sold (in shares)	2,792,000	0	3,277,000
Weighted average price per share (in dollars per share)	$ 38.32		$ 38.16
Gross proceeds	$ 106,992		$ 125,058