Business Segments - Summary on Reconciliation of Results Under Ifrs to Us Gaap (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2021		Mar. 31, 2020	Dec. 31, 2020
Operating Statistics [Line Items]
Net Interest Income		$ 493		$ 519
Group Reporting Basis Other operating income		410		371
Group Reporting Basis Total operating income		903		890
Expected credit losses / provision for credit losses		(91)		379
Group Reporting Basis Revenues		994		511
Operating Expenses		669		1,509
Group Reporting Basis Profit (Loss) Before Income Tax		325		(998)
Total Assets		219,949		269,609
Group Reporting Basis Total loans, net		58,974		78,655
Group Reporting Basis Goodwill		358		358
Group Reporting Basis Total deposits		141,139		124,315
Net interest income		531		504
Other operating income	[1]	384		387
Total operating income		915		891
Provision for credit losses		(227)		726
Total income		1,142		165
Operating expenses		682		1,605
Income (loss) before income tax		460		(1,440)
Total assets		194,856	[2]	205,077	$ 196,434	[2]
Total loans, net		60,673		80,797	61,073
Goodwill		458		458	458
Total deposits		148,582		136,723	$ 145,150
Group Reporting Basis Adjustments [Member]
Operating Statistics [Line Items]
Net Interest Income	[3]	4		3
Group Reporting Basis Other operating income	[3]	5		(4)
Group Reporting Basis Total operating income	[3]	9		(1)
Expected credit losses / provision for credit losses	[3]	(136)		347
Group Reporting Basis Revenues	[3]	145		(348)
Operating Expenses	[3]	10		94
Group Reporting Basis Profit (Loss) Before Income Tax	[3]	135		(442)
Total Assets	[3]	(25,093)		(64,532)
Group Reporting Basis Total loans, net	[3]	(1,367)		(4,596)
Group Reporting Basis Goodwill	[3]	100		100
Group Reporting Basis Total deposits	[3]	(4,472)		(7,066)
Group Reporting Basis Reclassifications [Member]
Operating Statistics [Line Items]
Net Interest Income	[4]	34		(18)
Group Reporting Basis Other operating income	[4]	(31)		20
Group Reporting Basis Total operating income	[4]	3		2
Expected credit losses / provision for credit losses	[4]	0		0
Group Reporting Basis Revenues	[4]	3		2
Operating Expenses	[4]	3		2
Group Reporting Basis Profit (Loss) Before Income Tax	[4]	0		0
Total Assets	[4]	0		0
Group Reporting Basis Total loans, net	[4]	3,066		6,738
Group Reporting Basis Goodwill	[4]	0		0
Group Reporting Basis Total deposits	[4]	11,915		19,474
WPB [Member]
Operating Statistics [Line Items]
Net Interest Income		209		232
Group Reporting Basis Other operating income		87		74
Group Reporting Basis Total operating income		296		306
Expected credit losses / provision for credit losses		(2)		140
Group Reporting Basis Revenues		298		166
Operating Expenses		287		1,035
Group Reporting Basis Profit (Loss) Before Income Tax		11		(869)
Total Assets		63,747		58,246
Group Reporting Basis Total loans, net		24,513		24,478
Group Reporting Basis Goodwill		0		0
Group Reporting Basis Total deposits		49,578		48,800
CMB [Member]
Operating Statistics [Line Items]
Net Interest Income		190		210
Group Reporting Basis Other operating income		66		58
Group Reporting Basis Total operating income		256		268
Expected credit losses / provision for credit losses		(38)		123
Group Reporting Basis Revenues		294		145
Operating Expenses		150		147
Group Reporting Basis Profit (Loss) Before Income Tax		144		(2)
Total Assets		44,170		42,882
Group Reporting Basis Total loans, net		22,785		30,615
Group Reporting Basis Goodwill		358		358
Group Reporting Basis Total deposits		42,874		32,844
GB&M [Member]
Operating Statistics [Line Items]
Net Interest Income		95		90
Group Reporting Basis Other operating income		246		263
Group Reporting Basis Total operating income		341		353
Expected credit losses / provision for credit losses		(51)		116
Group Reporting Basis Revenues		392		237
Operating Expenses		201		192
Group Reporting Basis Profit (Loss) Before Income Tax		191		45
Total Assets		110,524		165,838
Group Reporting Basis Total loans, net		11,676		23,562
Group Reporting Basis Goodwill		0		0
Group Reporting Basis Total deposits		48,687		42,671
CC [Member]
Operating Statistics [Line Items]
Net Interest Income		(1)		(13)
Group Reporting Basis Other operating income		11		(24)
Group Reporting Basis Total operating income		10		(37)
Expected credit losses / provision for credit losses		0		0
Group Reporting Basis Revenues		10		(37)
Operating Expenses		31		135
Group Reporting Basis Profit (Loss) Before Income Tax		(21)		(172)
Total Assets		1,508		2,643
Group Reporting Basis Total loans, net		0		0
Group Reporting Basis Goodwill		0		0
Group Reporting Basis Total deposits		$ 0		$ 0

[1] See Note 15, "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.

[4] Represents differences in financial statement presentation between U.S. GAAP and the Group Reporting Basis