Derivatives (Outstanding Derivative Balances) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Derivatives And Hedging Activities [Line Items]
Gross Derivative Balance	$ (214)	$ (329)
Netting	0	0
Cash Collateral	148	165
Total Derivative Balance	(66)	(164)
Other Current Assets [Member]
Derivatives And Hedging Activities [Line Items]
Gross Derivative Balance	31	48
Netting	(12)	(25)
Cash Collateral	19	36
Total Derivative Balance	38	59
Other Noncurrent Assets [Member]
Derivatives And Hedging Activities [Line Items]
Gross Derivative Balance	66	99
Netting	(5)	(11)
Cash Collateral	0	0
Total Derivative Balance	61	88
Other Current Liabilities [Member]
Derivatives And Hedging Activities [Line Items]
Gross Derivative Balance	(162)	(255)
Netting	12	25
Cash Collateral	106	115
Total Derivative Balance	(44)	(115)
Other Noncurrent Liabilities [Member]
Derivatives And Hedging Activities [Line Items]
Gross Derivative Balance	(149)	(221)
Netting	5	11
Cash Collateral	23	14
Total Derivative Balance	$ (121)	$ (196)