Fair Value Measurements - Assets Measured at Fair Value on Nonrecurring Basis (Detail) (Nonrecurring [Member], USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	$ 52	$ 149
Loans held for sale	10	15
Accrued income and other assets	37	44
Total assets on a nonrecurring basis at fair value	99	208
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accrued income and other assets	15	19
Total assets on a nonrecurring basis at fair value	15	19
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	52	149
Loans held for sale	10	15
Accrued income and other assets	22	25
Total assets on a nonrecurring basis at fair value	$ 84	$ 189