Credit Agreements (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014
Credit Agreements Details [Line Items]
Total amount of credit facilities available	$ 843
Outstanding Unsecured Letter Of Credit [Member]
Credit Agreements Details [Line Items]
Letters Of Credit Outstanding Amount	122
Outstanding Secured Letter Of Credit [Member]
Credit Agreements Details [Line Items]
Letters Of Credit Outstanding Amount	420
Combined Credit Facility [Member]
Credit Agreements Details [Line Items]
Total amount of credit facilities available	300
Maturity date of facility	Nov. 14, 2015
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