Fair Value Measurements - Assets Measured at Fair Value on Nonrecurring Basis (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
ASSETS MEASURED ON A NONRECURRING BASIS
Loans held for sale	$ 0	$ 0	$ 0
Nonrecurring [Member]
ASSETS MEASURED ON A NONRECURRING BASIS
Impaired loans	15	5	1
Loans held for sale	0	0	0
Accrued income and other assets	18	13	13
Total assets at fair value	33	18	14
Level 1 [Member] | Nonrecurring [Member]
ASSETS MEASURED ON A NONRECURRING BASIS
Loans held for sale	0	0	0
Level 2 [Member] | Nonrecurring [Member]
ASSETS MEASURED ON A NONRECURRING BASIS
Loans held for sale	0	0	0
Level 3 [Member] | Nonrecurring [Member]
ASSETS MEASURED ON A NONRECURRING BASIS
Impaired loans	15	5	1
Loans held for sale	0	0	0
Accrued income and other assets	18	13	13
Total assets at fair value	$ 33	$ 18	$ 14