Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Nonrecurring Basis (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Sep. 30, 2018
ASSETS MEASURED ON A NONRECURRING BASIS
Tax credit carryforward, provisional impairment charges	$ 31
Fair Value, Measurements, Nonrecurring [Member]
ASSETS MEASURED ON A NONRECURRING BASIS
Impaired loans and leases	9	$ 97
Accrued income and other assets	138	24
Total assets at fair value	147	121
Level 1 [Member] | Fair Value, Measurements, Nonrecurring [Member]
ASSETS MEASURED ON A NONRECURRING BASIS
Impaired loans and leases	0	0
Accrued income and other assets	0	0
Total assets at fair value	0	0
Level 2 [Member] | Fair Value, Measurements, Nonrecurring [Member]
ASSETS MEASURED ON A NONRECURRING BASIS
Impaired loans and leases	0	0
Accrued income and other assets	5	0
Total assets at fair value	5	0
Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member]
ASSETS MEASURED ON A NONRECURRING BASIS
Impaired loans and leases	9	97
Accrued income and other assets	133	24
Total assets at fair value	142	$ 121
LIHTC And Historic Tax Credit Investments [Member]
ASSETS MEASURED ON A NONRECURRING BASIS
Tax credit carryforward, amount	$ 119