Business Segments - Summary on Reconciliation of Results Under Ifrs to Us Gaap (Detail) - USD ($) $ in Millions		3 Months Ended										12 Months Ended
		Dec. 31, 2020		Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019		Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020		Dec. 31, 2019		Dec. 31, 2018
Operating Statistics [Line Items]
Net Interest Income												$ 2,026		$ 2,303		$ 2,559
Group Reporting Basis Other operating income												1,701		1,537		1,604
Group Reporting Basis Total operating income												3,727		3,840		4,163
Expected credit losses / provision for credit losses												622
Expected credit losses/provision for credit losses														171		(199)
Group Reporting Basis Revenues												3,105		3,669		4,362
Operating Expenses												4,034		3,313		3,633
Group Reporting Basis Profit (Loss) Before Income Tax												(929)		356		729
Total Assets		$ 229,677					$ 214,223					229,677		214,223		203,375
Group Reporting Basis Total loans, net		59,489					66,449					59,489		66,449		67,040
Group Reporting Basis Goodwill		358					1,051					358		1,051		1,260
Group Reporting Basis Total deposits		137,037					103,589					137,037		103,589		97,371
Net interest income		584		$ 548	$ 538	$ 504	507		$ 503	$ 540	$ 562	2,174		2,112		2,252
Other operating income		328		356	464	387	542		486	436	390	1,535	[1]	1,854	[1]	1,899	[1]
Total operating income												3,709		3,966		4,151
Provision for credit losses		(30)		(105)	219	726						810		195		(73)
Provision for credit losses							(3)		94	46	58	736		195		(73)
Total income												2,899		3,771		4,224
Operating expenses		764		785	727	1,605	871		1,106	774	750	3,881		3,501		3,638
Income (loss) before income tax		178		$ 224	$ 56	$ (1,440)	181		$ (211)	$ 156	$ 144	(982)		270		586
Total assets		196,434	[2]				175,375	[2]				196,434	[2]	175,375	[2]	172,448
Total loans, net		61,073					67,916					61,073		67,916		68,437
Goodwill		458					1,242					458		1,242		1,607
Total deposits		145,150					119,693					145,150		119,693		110,955
Group Reporting Basis Adjustments [Member]
Operating Statistics [Line Items]
Net Interest Income	[3]											9		13		(16)
Group Reporting Basis Other operating income	[3]											(46)		102		(9)
Group Reporting Basis Total operating income	[3]											(37)		115		(25)
Expected credit losses / provision for credit losses	[3]											188
Expected credit losses/provision for credit losses	[3]													32		115
Group Reporting Basis Revenues	[3]											(225)		83		(140)
Operating Expenses	[3]											(172)		169		3
Group Reporting Basis Profit (Loss) Before Income Tax	[3]											(53)		(86)		(143)
Total Assets	[3]	(33,243)					(38,848)					(33,243)		(38,848)		(30,927)
Group Reporting Basis Total loans, net	[3]	(2,307)					(2,965)					(2,307)		(2,965)		(2,428)
Group Reporting Basis Goodwill	[3]	100					191					100		191		347
Group Reporting Basis Total deposits	[3]	(4,253)					(4,747)					(4,253)		(4,747)		(2,017)
Group Reporting Basis Reclassifications [Member]
Operating Statistics [Line Items]
Net Interest Income	[4]											139		(204)		(291)
Group Reporting Basis Other operating income	[4]											(120)		215		304
Group Reporting Basis Total operating income	[4]											19		11		13
Expected credit losses / provision for credit losses	[4]											0
Expected credit losses/provision for credit losses	[4]													(8)		11
Group Reporting Basis Revenues	[4]											19		19		2
Operating Expenses	[4]											19		19		2
Group Reporting Basis Profit (Loss) Before Income Tax	[4]											0		0		0
Total Assets	[4]	0					0					0		0		0
Group Reporting Basis Total loans, net	[4]	3,891					4,432					3,891		4,432		3,825
Group Reporting Basis Goodwill	[4]	0					0					0		0		0
Group Reporting Basis Total deposits	[4]	12,366					20,851					12,366		20,851		15,601
WPB [Member] | Operating Segments [Member]
Operating Statistics [Line Items]
Net Interest Income												837		1,018		1,132
Group Reporting Basis Other operating income												368		330		394
Group Reporting Basis Total operating income												1,205		1,348		1,526
Expected credit losses / provision for credit losses												189
Expected credit losses/provision for credit losses														115		35
Group Reporting Basis Revenues												1,016		1,233		1,491
Operating Expenses												2,203		1,645		1,538
Group Reporting Basis Profit (Loss) Before Income Tax												(1,187)		(412)		(47)
Total Assets		59,200					50,851					59,200		50,851		53,298
Group Reporting Basis Total loans, net		24,048					24,243					24,048		24,243		23,209
Group Reporting Basis Goodwill		0					693					0		693		902
Group Reporting Basis Total deposits		50,000					44,553					50,000		44,553		41,806
CMB [Member] | Operating Segments [Member]
Operating Statistics [Line Items]
Net Interest Income												821		829		821
Group Reporting Basis Other operating income												234		239		234
Group Reporting Basis Total operating income												1,055		1,068		1,055
Expected credit losses / provision for credit losses												293
Expected credit losses/provision for credit losses														55		(42)
Group Reporting Basis Revenues												762		1,013		1,097
Operating Expenses												587		581		595
Group Reporting Basis Profit (Loss) Before Income Tax												175		432		502
Total Assets		36,354					34,241					36,354		34,241		34,214
Group Reporting Basis Total loans, net		23,377					24,647					23,377		24,647		24,105
Group Reporting Basis Goodwill		358					358					358		358		358
Group Reporting Basis Total deposits		42,369					29,986					42,369		29,986		23,938
GB&M [Member] | Operating Segments [Member]
Operating Statistics [Line Items]
Net Interest Income												398		521		712
Group Reporting Basis Other operating income												970		900		788
Group Reporting Basis Total operating income												1,368		1,421		1,500
Expected credit losses / provision for credit losses												140
Expected credit losses/provision for credit losses														1		(192)
Group Reporting Basis Revenues												1,228		1,420		1,692
Operating Expenses												841		849		858
Group Reporting Basis Profit (Loss) Before Income Tax												387		571		834
Total Assets		132,363					126,710					132,363		126,710		112,911
Group Reporting Basis Total loans, net		12,064					17,559					12,064		17,559		19,726
Group Reporting Basis Goodwill		0					0					0		0		0
Group Reporting Basis Total deposits		44,668					29,050					44,668		29,050		31,627
CC [Member] | Operating Segments [Member]
Operating Statistics [Line Items]
Net Interest Income												(30)		(65)		(106)
Group Reporting Basis Other operating income												129		68		188
Group Reporting Basis Total operating income												99		3		82
Expected credit losses / provision for credit losses												0
Expected credit losses/provision for credit losses														0		0
Group Reporting Basis Revenues												99		3		82
Operating Expenses												403		238		642
Group Reporting Basis Profit (Loss) Before Income Tax												(304)		(235)		(560)
Total Assets		1,760					2,421					1,760		2,421		2,952
Group Reporting Basis Total loans, net		0					0					0		0		0
Group Reporting Basis Goodwill		0					0					0		0		0
Group Reporting Basis Total deposits		$ 0					$ 0					$ 0		$ 0		$ 0

[1] See Note 24, "Business Segments," for a reconciliation of total other revenues on a U.S. GAAP basis to other operating income for each business segment under the Group Reporting Basis.

[2] Assets

[3] Represents adjustments associated with differences between U.S. GAAP and the Group Reporting Basis. These adjustments, which are more fully described above, consist of the following:

[4] Net
Interest
IncomeOther
RevenuesProvision
for Credit
LossesOperating
ExpensesProfit (Loss)
before Income
Tax ExpenseTotal
Assets(in millions)December 31, 2020Deposit incentives$14 $— $— $21 $(7)$(28)Derivatives— — — — — (33,163)Expected credit losses(4)— 200 — (204)(357)Goodwill impairment— — — 91 (91)100 Leases23 — — 14 9 33 Loan origination(24)(2)— (20)(6)18 Loans held for sale— (25)(12)— (13)— Other long-lived assets impairment— — — (264)264 201 Pension and other postretirement benefit costs— — — (15)15 — Renewable energy tax credit investments— (11)— — (11)2 Securities transferred to held-to-maturity— — — — — (38)Other— (8)— 1 (9)(11)Total adjustments$9 $(46)$188 $(172)$(53)$(33,243)December 31, 2019Debt extinguishment$— $123 $— $— $123 $— Deposit incentives15 — — 32 (17)(23)Derivatives— — — — — (38,651)Expected credit losses(3)(1)32 — (36)(357)Goodwill impairment— — — 156 (156)191 Leases25 — — 8 17 15 Loan origination(27)— — (19)(8)38 Loans held for sale2 (1)— — 1 (5)Pension and other postretirement benefit costs— — — (12)12 — Renewable energy tax credit investments— (7)— — (7)3 Securities transferred to held-to-maturity— — — — — (65)Other1 (12)— 4 (15)6 Total adjustments$13 $102 $32 $169 $(86)$(38,848)December 31, 2018Deposit incentives$9 $— $— $20 $(11)$(10)Derivatives— — — — — (30,866)Expected credit losses(6)— 118 — (124)(386)Goodwill— — — — — 347 Loan origination(29)(3)— (14)(18)45 Loans held for sale9 6 (3)— 18 45 Pension and other postretirement benefit costs— — — 8 (8)(44)Renewable energy tax credit investments— (10)— — (10)3 Securities transferred to held-to-maturity— — — — — (76)Other1 (2)— (11)10 15 Total adjustments$(16)$(9)$115 $3 $(143)$(30,927)