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
	Mar. 31, 2016	Mar. 31, 2015
Class of Stock [Line Items]
Shares sold (in shares)	485	809
Weighted average price per share (in dollars per share)	$ 37.25	$ 36.29
Gross proceeds	$ 18,065	$ 29,360