Common Share of Beneficial Interest (Narrative ) (Details) - USD ($) shares in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2015	Feb. 28, 2015	Dec. 31, 2014	Apr. 18, 2013
Class of Stock [Line Items]
Shares issued (in shares)	123,934	123,934		122,489
ATM Equity Program [Member]
Class of Stock [Line Items]
Amount available for issuance, ATM equity program	$ 159,200,000	$ 159,200,000
Gross proceeds	$ 11,476,000	$ 40,836,000
ATM Equity Program [Member] | Maximum [Member]
Class of Stock [Line Items]
Amount available for issuance, ATM equity program			$ 200,000,000
Unsecured Revolving Credit Facility [Member]
Class of Stock [Line Items]
Maximum borrowing capacity under credit facility					$ 500,000,000