Derivatives (Outstanding Derivative Balances) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Derivatives And Hedging Activities [Line Items]
Gross Derivative Balance	$ (774)	$ (720)
Cash Collateral	516	478
Total Derivative Balance	(258)	(242)
Other Current Assets [Member]
Derivatives And Hedging Activities [Line Items]
Gross Derivative Balance	42	54
Netting	(27)	(39)
Cash Collateral	97	103
Total Derivative Balance	112	118
Other Noncurrent Assets [Member]
Derivatives And Hedging Activities [Line Items]
Gross Derivative Balance	96	113
Netting	(43)	(59)
Total Derivative Balance	53	54
Other Current Liabilities [Member]
Derivatives And Hedging Activities [Line Items]
Gross Derivative Balance	(515)	(489)
Netting	27	39
Cash Collateral	318	274
Total Derivative Balance	(170)	(176)
Other Noncurrent Liabilities [Member]
Derivatives And Hedging Activities [Line Items]
Gross Derivative Balance	(397)	(398)
Netting	43	59
Cash Collateral	101	101
Total Derivative Balance	$ (253)	$ (238)