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		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Class of Stock [Line Items]
Shares sold (in shares)	188	0	3,465	1,129
Weighted average price per share (in dollars per share)	$ 41.67	$ 0.00	$ 38.35	$ 36.18
Gross proceeds	$ 7,827	$ 0	$ 132,884	$ 40,836