Common Shares of Beneficial Interest (Narrative ) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Class of Stock [Line Items]
Value of shares approved to be repurchased	$ 200,000,000		$ 200,000,000
ATM Equity Program [Member]
Class of Stock [Line Items]
Amount available for issuance, ATM equity program	$ 0		$ 0
Shares sold (in shares)	0	188,000	0	3,465,000
ATM Equity Program [Member] | Maximum [Member]
Class of Stock [Line Items]
Amount available for issuance, ATM equity program	$ 250,000,000		$ 250,000,000