Common Share of Beneficial Interest (Narrative ) (Details) - USD ($)	1 Months Ended	3 Months Ended
	May. 02, 2016	Mar. 31, 2016	Mar. 31, 2015	Mar. 30, 2016	Dec. 31, 2015	May. 31, 2015	Feb. 28, 2015
Class of Stock [Line Items]
Number of shares approved to be repurchased		200,000,000
ATM Equity Program [Member]
Class of Stock [Line Items]
Shares sold (in shares)		485,000	809,000
Gross proceeds		$ 18,065,000	$ 29,360,000
ATM Equity Program [Member] | Maximum [Member]
Class of Stock [Line Items]
Amount available for issuance, ATM equity program							$ 200,000,000
Subsequent Event [Member] | ATM Equity Program [Member]
Class of Stock [Line Items]
Amount available for issuance, ATM equity program	$ 119,200,000
Shares sold (in shares)	582,907
Gross proceeds	$ 21,900,000
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	$ 500,000,000