Common Shares of Beneficial Interest (Narrative ) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	May 03, 2017	Aug. 31, 2016	Mar. 30, 2016	Oct. 31, 2015	Feb. 28, 2015
Class of Stock [Line Items]
Number of shares approved to be repurchased (in shares)						200,000,000
ATM Equity Program [Member]
Class of Stock [Line Items]
Shares sold (in shares)	0	485,000
ATM Equity Program [Member] | Maximum [Member]
Class of Stock [Line Items]
Amount available for issuance, ATM equity program	$ 250,000,000
Subsequent Event [Member] | ATM Equity Program [Member]
Class of Stock [Line Items]
Amount available for issuance, ATM equity program			$ 242,200,000
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