Reconciliation of Assets and Liabilities Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Level 3) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 130	$ 107	$ 222
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation Calculation [Abstract]
Included in earnings	440	166	176
Purchases		2
Settlements		(164)	(317)
Sales		5
Settlements	(318)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset And Liabilities Level 3 Transfers Net	26	24	26
Ending balance	278	130	107
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset And Liabilities Change In Unrealized Gains Losses Relating To Assets	255	(7)	46
Trading Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		6	13
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation Calculation [Abstract]
Included in earnings			3
Settlements			(10)
Sales		5
Ending balance	1	1	6
Residential Mortgage [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	65	46	26
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation Calculation [Abstract]
Included in earnings		4
Settlements		(9)	(6)
Settlements	(15)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset And Liabilities Level 3 Transfers Net	26	24	26
Ending balance	76	65	46
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset And Liabilities Change In Unrealized Gains Losses Relating To Assets		4
Interest Rate Contract
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	32	2	(2)
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation Calculation [Abstract]
Included in earnings	418	205	187
Purchases			(183)
Settlements		(175)
Settlements	(393)
Ending balance	57	32	2
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset And Liabilities Change In Unrealized Gains Losses Relating To Assets	233	32	60
Equity Contract
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	32	53	11
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation Calculation [Abstract]
Included in earnings	22	(43)	(14)
Purchases		2
Settlements		20	56
Settlements	90
Ending balance	144	32	53
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset And Liabilities Change In Unrealized Gains Losses Relating To Assets	22	(43)	(14)
Retained Interest
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance			174
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation Calculation [Abstract]
Settlements			$ (174)