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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Class of Stock [Line Items]
Shares sold (in shares)	0	188,000	0	3,465,000
Weighted average price per share (in dollars per share)		$ 41.67		$ 38.35
Gross proceeds		$ 7,827		$ 132,884