Credit Agreements (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Credit Agreements Details [Line Items]
Total amount of credit facilities available	$ 1,440
[OutstandingUnsecuredLetterOfCreditMember]
Credit Agreements Details [Line Items]
Letters of credit issued as of balance sheet date	246
[OutstandingSecuredLetterOfCreditMember]
Credit Agreements Details [Line Items]
Letters of credit issued as of balance sheet date	524
[UnsecuredSyndicatedCreditFacilityBMember]
Credit Agreements Details [Line Items]
Total amount of credit facilities available	750
Maturity date of facility	7/16/2013
Accordion feature	250
Total borrowing capacity with accordion	1,000
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	250
Maturity date of facility	11/14/2012
Maturity date of facility, extension	11/14/2013
Modification date		11/14/2011
Amount of unsecured borrowing capacity	100
[CommittedSecuredLetterOfCreditFacilityAMember]
Credit Agreements Details [Line Items]
Total amount of credit facilities available	150
[CommittedSecuredLetterOfCreditFacilityBMember]
Credit Agreements Details [Line Items]
Total amount of credit facilities available	$ 200
Maturity date of facility	12/31/2014