Fair Value Measurements - Assets Measured at Fair Value on Nonrecurring Basis (Detail) (Nonrecurring [Member], USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
ASSETS MEASURED ON A NONRECURRING BASIS
Impaired loans	$ 26	$ 25	$ 52
Loans held for sale		9	10
Accrued income and other assets	11	22	37
Total assets at fair value	37	56	99
Level 2 [Member]
ASSETS MEASURED ON A NONRECURRING BASIS
Accrued income and other assets	1	2	15
Total assets at fair value	1	2	15
Level 3 [Member]
ASSETS MEASURED ON A NONRECURRING BASIS
Impaired loans	26	25	52
Loans held for sale		9	10
Accrued income and other assets	10	20	22
Total assets at fair value	$ 36	$ 54	$ 84