Differences between Aggregate Fair Value, Carrying Amount of MLHFS and Aggregate Unpaid Principal Amount (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Carrying Amount
Fair Value, Option, Quantitative Disclosures [Line Items]
Total loans	$ 9,815	$ 6,925
Nonaccrual loans	8	10
Loans 90 days or more past due	2	3
Aggregate Unpaid Principal
Fair Value, Option, Quantitative Disclosures [Line Items]
Total loans	9,215	6,635
Nonaccrual loans	13	15
Loans 90 days or more past due	3	4
Excess of Carrying Amount Over (Under) Unpaid Principal
Fair Value, Option, Quantitative Disclosures [Line Items]
Total loans	600	290
Nonaccrual loans	(5)	(5)
Loans 90 days or more past due	$ (1)	$ (1)