Credit Agreements (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($)
Credit Agreements Details [Line Items]
Total amount of credit facilities available	$ 817
Outstanding Unsecured Letter Of Credit [Member]
Credit Agreements Details [Line Items]
Letters Of Credit Outstanding Amount	113
Outstanding Secured Letter Of Credit [Member]
Credit Agreements Details [Line Items]
Letters Of Credit Outstanding Amount	375
Combined Credit Facility [Member]
Credit Agreements Details [Line Items]
Total amount of credit facilities available	$ 300
Maturity date of facility	Nov. 14, 2016
Amount of unsecured borrowing capacity	$ 100
Committed Secured Letter Of Credit Facility A [Member]
Credit Agreements Details [Line Items]
Total amount of credit facilities available	$ 300
Maturity date of facility	Dec. 31, 2018
Committed Secured Letter Of Credit Facility B [Member]
Credit Agreements Details [Line Items]
Total amount of credit facilities available	$ 140
Maturity date of facility	Dec. 31, 2017