Derivatives (Outstanding Derivative Balances) (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Derivatives And Hedging Activities [Line Items]
Gross Derivative Balance	$ (254)	$ (329)
Netting	0	0
Cash Collateral	201	165
Total Derivative Balance	(53)	(164)
Other Current Assets [Member]
Derivatives And Hedging Activities [Line Items]
Gross Derivative Balance	31	48
Netting	(11)	(25)
Cash Collateral	28	36
Total Derivative Balance	48	59
Other Noncurrent Assets [Member]
Derivatives And Hedging Activities [Line Items]
Gross Derivative Balance	71	99
Netting	(5)	(11)
Cash Collateral	0	0
Total Derivative Balance	66	88
Other Current Liabilities [Member]
Derivatives And Hedging Activities [Line Items]
Gross Derivative Balance	(188)	(255)
Netting	11	25
Cash Collateral	133	115
Total Derivative Balance	(44)	(115)
Other Noncurrent Liabilities [Member]
Derivatives And Hedging Activities [Line Items]
Gross Derivative Balance	(168)	(221)
Netting	5	11
Cash Collateral	40	14
Total Derivative Balance	$ (123)	$ (196)