Loans and Allowance for Credit Losses - Additional Detail of Allowance for Credit Losses and Related Loan Balances by Portfolio Class (Detail) - USD ($) $ in Millions	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Allowance Balance at End of Period
Loans individually evaluated for impairment	$ 22		$ 24
TDRs collectively evaluated for impairment	223		225
Other loans collectively evaluated for impairment	4,221		4,176
Loans acquired with deteriorated credit quality	15		16
Total allowance for credit losses	4,481	$ 4,466	4,441	$ 4,426	$ 4,411	$ 4,417
Loan Balance at End of Period
Loans individually evaluated for impairment	380		348
TDRs collectively evaluated for impairment	3,940		3,960
Other loans collectively evaluated for impairment	290,024		282,158
Loans acquired with deteriorated credit quality	294		344
Total loans	294,638		286,810
Commercial [Member]
Allowance Balance at End of Period
Loans individually evaluated for impairment	19		16
TDRs collectively evaluated for impairment	20		15
Other loans collectively evaluated for impairment	1,451		1,423
Total allowance for credit losses	1,490	1,464	1,454	1,413	1,391	1,372
Loan Balance at End of Period
Loans individually evaluated for impairment	340		262
TDRs collectively evaluated for impairment	178		151
Other loans collectively evaluated for impairment	104,136		102,031
Total loans	104,654		102,444
Commercial Real Estate [Member]
Allowance Balance at End of Period
Loans individually evaluated for impairment	3		8
TDRs collectively evaluated for impairment	4		3
Other loans collectively evaluated for impairment	784		788
Loans acquired with deteriorated credit quality			1
Total allowance for credit losses	791	794	800	809	812	831
Loan Balance at End of Period
Loans individually evaluated for impairment	40		86
TDRs collectively evaluated for impairment	142		129
Other loans collectively evaluated for impairment	39,052		39,297
Loans acquired with deteriorated credit quality	34		27
Total loans	39,268		39,539
Residential Mortgages [Member]
Allowance Balance at End of Period
TDRs collectively evaluated for impairment	107		126
Other loans collectively evaluated for impairment	309		314
Loans acquired with deteriorated credit quality	15		15
Total allowance for credit losses	431	438	455	440	436	449
Loan Balance at End of Period
TDRs collectively evaluated for impairment	3,168		3,252
Other loans collectively evaluated for impairment	65,950		61,465
Loans acquired with deteriorated credit quality	260		317
Total loans	69,378		65,034
Other Retail [Member]
Allowance Balance at End of Period
TDRs collectively evaluated for impairment	12		12
Other loans collectively evaluated for impairment	624		618
Total allowance for credit losses	636	638	630	647	667	678
Loan Balance at End of Period
TDRs collectively evaluated for impairment	190		183
Other loans collectively evaluated for impairment	57,258		56,247
Total loans	57,448		56,430
Credit Card [Member]
Allowance Balance at End of Period
TDRs collectively evaluated for impairment	80		69
Other loans collectively evaluated for impairment	1,053		1,033
Total allowance for credit losses	1,133	$ 1,132	1,102	$ 1,097	$ 1,082	$ 1,056
Loan Balance at End of Period
TDRs collectively evaluated for impairment	262		245
Other loans collectively evaluated for impairment	23,628		23,118
Total loans	$ 23,890		$ 23,363