Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Nonrecurring Basis (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
ASSETS MEASURED ON A NONRECURRING BASIS
Loans held for sale	$ 0	$ 0
Nonrecurring [Member]
ASSETS MEASURED ON A NONRECURRING BASIS
Impaired loans	11,000,000	0
Loans held for sale	0	0
Accrued income and other assets	11,000,000	7,000,000
Total assets on a recurring basis at fair value	22,000,000	7,000,000
Transfer of commercial and residential loans and leases from HFS to HTM	35,000,000	62,000,000
Nonrecurring [Member] | Level 1 [Member]
ASSETS MEASURED ON A NONRECURRING BASIS
Impaired loans	0	0
Loans held for sale	0	0
Accrued income and other assets	0	0
Total assets on a recurring basis at fair value	0	0
Nonrecurring [Member] | Level 2 [Member]
ASSETS MEASURED ON A NONRECURRING BASIS
Impaired loans	0	0
Loans held for sale	0	0
Accrued income and other assets	0	0
Total assets on a recurring basis at fair value	0	0
Nonrecurring [Member] | Level 3 [Member]
ASSETS MEASURED ON A NONRECURRING BASIS
Impaired loans	11,000,000	0
Loans held for sale	0	0
Accrued income and other assets	11,000,000	7,000,000
Total assets on a recurring basis at fair value	$ 22,000,000	$ 7,000,000