Common Shares of Beneficial Interest (Narrative ) (Details) - USD ($)	3 Months Ended	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Aug. 02, 2017	Aug. 31, 2016	Mar. 30, 2016
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)	2,792,000	0	3,277,000
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