Common Shares of Beneficial Interest (Narrative ) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Nov. 09, 2016	Aug. 31, 2016	Mar. 30, 2016	Dec. 31, 2015	May 31, 2015	Mar. 31, 2015	Feb. 28, 2015
Class of Stock [Line Items]
Number of shares approved to be repurchased (in shares)	200,000,000		200,000,000
ATM Equity Program [Member]
Class of Stock [Line Items]
Amount available for issuance, ATM equity program						$ 34,100,000
Shares sold (in shares)	188,000	0	3,465,000	1,129,000
Gross proceeds	$ 7,827,000	$ 0	$ 132,884,000	$ 40,836,000
ATM Equity Program [Member] | Maximum [Member]
Class of Stock [Line Items]
Amount available for issuance, ATM equity program	$ 0		$ 0			250,000,000					$ 200,000,000
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	$ 500,000,000	$ 500,000,000	$ 500,000,000