Credit Agreements (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Credit Agreements Details [Line Items]
Total amount of credit facilities available	$ 1,467
[OutstandingUnsecuredLetterOfCreditMember]
Credit Agreements Details [Line Items]
Letters Of Credit Outstanding Amount	189
[OutstandingSecuredLetterOfCreditMember]
Credit Agreements Details [Line Items]
Letters Of Credit Outstanding Amount	515
[UnsecuredSyndicatedCreditFacilityBMember]
Credit Agreements Details [Line Items]
Total amount of credit facilities available	500	750
Maturity date of facility	Jul. 16, 2013
Accordion feature	250
Total borrowing capacity with accordion	750
Revolving credit amount	375
Term of credit facility	3
Borrowings at balance sheet date	0
[CombinedCreditFacilityMember]
Credit Agreements Details [Line Items]
Total amount of credit facilities available	300	300	250
Maturity date of facility	Nov. 14, 2013
Modification date	11/14/2011
Amount of unsecured borrowing capacity		100
[CommittedSecuredLetterOfCreditFacilityAMember]
Credit Agreements Details [Line Items]
Total amount of credit facilities available	250
Maturity date of facility	Dec. 04, 2015
[CommittedSecuredLetterOfCreditFacilityBMember]
Credit Agreements Details [Line Items]
Total amount of credit facilities available	$ 200
Maturity date of facility	Dec. 31, 2014