Derivatives (Outstanding Derivative Balances) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Derivatives And Hedging Activities [Line Items]
Gross Derivative Balance	$ (393)	$ (720)
Netting	0	0
Cash Collateral	211	478
Total Derivative Balance	(182)	(242)
Other Current Assets [Member]
Derivatives And Hedging Activities [Line Items]
Gross Derivative Balance	52	54
Netting	(37)	(39)
Cash Collateral	75	103
Total Derivative Balance	90	118
Other Noncurrent Assets [Member]
Derivatives And Hedging Activities [Line Items]
Gross Derivative Balance	94	113
Netting	(36)	(59)
Cash Collateral	0	0
Total Derivative Balance	58	54
Other Current Liabilities [Member]
Derivatives And Hedging Activities [Line Items]
Gross Derivative Balance	(280)	(489)
Netting	37	39
Cash Collateral	119	274
Total Derivative Balance	(124)	(176)
Other Noncurrent Liabilities [Member]
Derivatives And Hedging Activities [Line Items]
Gross Derivative Balance	(259)	(398)
Netting	36	59
Cash Collateral	17	101
Total Derivative Balance	$ (206)	$ (238)