Common Share of Beneficial Interest (Narrative ) (Details) - USD ($)	Nov. 02, 2015	Sep. 30, 2015	May. 31, 2015	Mar. 31, 2015	Feb. 28, 2015	Apr. 18, 2013
ATM Equity Program [Member]
Class of Stock [Line Items]
Amount available for issuance, ATM equity program		$ 159,200,000
ATM Equity Program [Member] | Maximum [Member]
Class of Stock [Line Items]
Amount available for issuance, ATM equity program					$ 200,000,000
Subsequent Event [Member]
Class of Stock [Line Items]
Number of shares approved to be repurchased	200,000,000
Unsecured Revolving Credit Facility [Member]
Class of Stock [Line Items]
Maximum borrowing capacity under credit facility			$ 500,000,000	$ 500,000,000	$ 500,000,000	$ 500,000,000