Common Shares of Beneficial Interest (Schedule of ATM Equity Program) (Details) - ATM Equity Program [Member] - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Class of Stock [Line Items]
Shares sold (in shares)	0	485,000
Weighted average price per share (in dollars per share)		$ 37.25
Gross proceeds		$ 18,065