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, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Allowance Balance at End of Period
Loans individually evaluated for impairment	$ 38		$ 27
TDRs collectively evaluated for impairment	221		237
Other loans collectively evaluated for impairment	4,146		4,118
Loans acquired with deteriorated credit quality	21		35
Total allowance for credit losses	4,426	$ 4,411	4,417	$ 4,407	$ 4,377	$ 4,357
Loan Balance at End of Period
Loans individually evaluated for impairment	386		408
TDRs collectively evaluated for impairment	4,060		4,269
Other loans collectively evaluated for impairment	276,305		273,669
Loans acquired with deteriorated credit quality	710		2,086
Total loans	280,061		277,311
Total loans	281,461		280,432
Commercial [Member]
Allowance Balance at End of Period
Loans individually evaluated for impairment	30		23
TDRs collectively evaluated for impairment	13		14
Other loans collectively evaluated for impairment	1,370		1,335
Total allowance for credit losses	1,413	1,391	1,372	1,383	1,395	1,450
Loan Balance at End of Period
Loans individually evaluated for impairment	242		337
TDRs collectively evaluated for impairment	157		148
Other loans collectively evaluated for impairment	98,874		97,076
Total loans	99,273		97,561
Commercial Real Estate [Member]
Allowance Balance at End of Period
Loans individually evaluated for impairment	8		4
TDRs collectively evaluated for impairment	4		4
Other loans collectively evaluated for impairment	795		818
Loans acquired with deteriorated credit quality	2		5
Total allowance for credit losses	809	812	831	851	856	812
Loan Balance at End of Period
Loans individually evaluated for impairment	144		71
TDRs collectively evaluated for impairment	149		145
Other loans collectively evaluated for impairment	39,633		40,174
Loans acquired with deteriorated credit quality	40		73
Total loans	39,966		40,463
Residential Mortgages [Member]
Allowance Balance at End of Period
TDRs collectively evaluated for impairment	124		139
Other loans collectively evaluated for impairment	316		310
Total allowance for credit losses	440	436	449	450	455	510
Loan Balance at End of Period
TDRs collectively evaluated for impairment	3,322		3,524
Other loans collectively evaluated for impairment	59,581		56,258
Loans acquired with deteriorated credit quality	1		1
Total loans	62,904		59,783
Other Retail [Member]
Allowance Balance at End of Period
TDRs collectively evaluated for impairment	15		19
Other loans collectively evaluated for impairment	632		659
Total allowance for credit losses	647	667	678	672	648	617
Loan Balance at End of Period
TDRs collectively evaluated for impairment	183		186
Other loans collectively evaluated for impairment	55,866		57,138
Total loans	56,049		57,324
Credit Card [Member]
Allowance Balance at End of Period
TDRs collectively evaluated for impairment	64		60
Other loans collectively evaluated for impairment	1,033		996
Total allowance for credit losses	1,097	1,082	1,056	1,019	990	934
Loan Balance at End of Period
TDRs collectively evaluated for impairment	239		230
Other loans collectively evaluated for impairment	21,630		21,950
Total loans	21,869		22,180
Total Loans, Excluding Covered Loans [Member]
Allowance Balance at End of Period
Loans individually evaluated for impairment	38		27
TDRs collectively evaluated for impairment	220		236
Other loans collectively evaluated for impairment	4,146		4,118
Loans acquired with deteriorated credit quality	2		5
Total allowance for credit losses	4,406	4,388	4,386	4,375	4,344	4,323
Loan Balance at End of Period
Loans individually evaluated for impairment	386		408
TDRs collectively evaluated for impairment	4,050		4,233
Other loans collectively evaluated for impairment	275,584		272,596
Loans acquired with deteriorated credit quality	41		74
Total loans	280,061		277,311
Covered Loans [Member]
Allowance Balance at End of Period
TDRs collectively evaluated for impairment	1		1
Loans acquired with deteriorated credit quality	19		30
Total allowance for credit losses	20	$ 23	31	$ 32	$ 33	$ 34
Loan Balance at End of Period
TDRs collectively evaluated for impairment	10		36
Other loans collectively evaluated for impairment	721		1,073
Loans acquired with deteriorated credit quality	669		2,012
Total loans	$ 1,400		$ 3,121