Business Segments - Summary on Reconciliation of Results Under Ifrs to Us Gaap (Detail) - USD ($) $ in Millions		3 Months Ended			9 Months Ended
		Sep. 30, 2021		Sep. 30, 2020	Sep. 30, 2021		Sep. 30, 2020		Dec. 31, 2020
Operating Statistics [Line Items]
Net interest income (expense)		$ 475		$ 485	$ 1,448		$ 1,520
Group Reporting Basis Other operating income (expense)		273		433	1,027		1,303
Group Reporting Basis Total operating income (expense)		748		918	2,475		2,823
Expected credit losses / provision for credit losses		(42)		(15)	(216)		600
Group Reporting Basis Revenues		790		933	2,691		2,223
Operating Expenses		654		1,054	2,088		3,295
Group Reporting Basis Profit (Loss) Before Income Tax		136		(121)	603		(1,072)
Total Assets		219,401		232,867	219,401		232,867
Group Reporting Basis Total loans, net		54,334		63,795	54,334		63,795
Group Reporting Basis Goodwill		358		358	358		358
Group Reporting Basis Total deposits		137,131		139,779	137,131		139,779
Net interest income (expense)		520		548	1,574		1,590
Other operating income	[1]	242		356	930		1,207
Total operating income		762		904	2,504		2,797
Provision for credit losses		(81)		(105)	(537)		840
Total income		843		1,009	3,041		1,957
Operating expenses		666		785	2,150		3,117
Income (loss) before income tax		177		224	891		(1,160)
Total assets		201,150	[2]	200,946	201,150	[2]	200,946		$ 196,434	[2]
Total loans, net		55,051		65,325	55,051		65,325
Goodwill		458		458	458		458		458
Total deposits		152,368		150,328	152,368		150,328		$ 145,150
Group Reporting Basis Adjustments [Member]
Operating Statistics [Line Items]
Net interest income (expense)	[3]	4		1	17		6
Group Reporting Basis Other operating income (expense)	[3]	4		(22)	(2)		(45)
Group Reporting Basis Total operating income (expense)	[3]	8		(21)	15		(39)
Expected credit losses / provision for credit losses	[3]	(39)		(90)	(321)		240
Group Reporting Basis Revenues	[3]	47		69	336		(279)
Operating Expenses	[3]	6		(276)	48		(191)
Group Reporting Basis Profit (Loss) Before Income Tax	[3]	41		345	288		(88)
Total Assets	[3]	(18,251)		(31,921)	(18,251)		(31,921)
Group Reporting Basis Total loans, net	[3]	(1,549)		(1,682)	(1,549)		(1,682)
Group Reporting Basis Goodwill	[3]	100		100	100		100
Group Reporting Basis Total deposits	[3]	(3,668)		(5,198)	(3,668)		(5,198)
Group Reporting Basis Reclassifications [Member]
Operating Statistics [Line Items]
Net interest income (expense)		41		62	109	[4]	64	[4]
Group Reporting Basis Other operating income (expense)		(35)		(55)	(95)	[4]	(51)	[4]
Group Reporting Basis Total operating income (expense)		6		7	14	[4]	13	[4]
Expected credit losses / provision for credit losses		0		0	0	[4]	0	[4]
Group Reporting Basis Revenues		6		7	14	[4]	13	[4]
Operating Expenses		6		7	14	[4]	13	[4]
Group Reporting Basis Profit (Loss) Before Income Tax		0		0	0	[4]	0	[4]
Total Assets	[4]	0		0	0		0
Group Reporting Basis Total loans, net	[4]	2,266		3,212	2,266		3,212
Group Reporting Basis Goodwill	[4]	0		0	0		0
Group Reporting Basis Total deposits	[4]	18,905		15,747	18,905		15,747
WPB [Member]
Operating Statistics [Line Items]
Net interest income (expense)		202		200	616		629
Group Reporting Basis Other operating income (expense)		57		110	215		283
Group Reporting Basis Total operating income (expense)		259		310	831		912
Expected credit losses / provision for credit losses		(35)		12	(36)		188
Group Reporting Basis Revenues		294		298	867		724
Operating Expenses		279		527	921		1,888
Group Reporting Basis Profit (Loss) Before Income Tax		15		(229)	(54)		(1,164)
Total Assets		69,021		56,728	69,021		56,728
Group Reporting Basis Total loans, net		22,046		23,821	22,046		23,821
Group Reporting Basis Goodwill		0		0	0		0
Group Reporting Basis Total deposits		39,762		48,477	39,762		48,477
CMB [Member]
Operating Statistics [Line Items]
Net interest income (expense)		200		202	580		616
Group Reporting Basis Other operating income (expense)		70		66	208		176
Group Reporting Basis Total operating income (expense)		270		268	788		792
Expected credit losses / provision for credit losses		24		(21)	(26)		283
Group Reporting Basis Revenues		246		289	814		509
Operating Expenses		134		150	437		439
Group Reporting Basis Profit (Loss) Before Income Tax		112		139	377		70
Total Assets		46,226		38,096	46,226		38,096
Group Reporting Basis Total loans, net		20,998		25,852	20,998		25,852
Group Reporting Basis Goodwill		358		358	358		358
Group Reporting Basis Total deposits		45,239		39,736	45,239		39,736
GBM [Member]
Operating Statistics [Line Items]
Net interest income (expense)		74		92	255		304
Group Reporting Basis Other operating income (expense)		184		217	613		743
Group Reporting Basis Total operating income (expense)		258		309	868		1,047
Expected credit losses / provision for credit losses		(31)		(6)	(154)		129
Group Reporting Basis Revenues		289		315	1,022		918
Operating Expenses		186		251	573		644
Group Reporting Basis Profit (Loss) Before Income Tax		103		64	449		274
Total Assets		102,543		136,478	102,543		136,478
Group Reporting Basis Total loans, net		11,290		14,122	11,290		14,122
Group Reporting Basis Goodwill		0		0	0		0
Group Reporting Basis Total deposits		52,130		51,566	52,130		51,566
CC [Member]
Operating Statistics [Line Items]
Net interest income (expense)		(1)		(9)	(3)		(29)
Group Reporting Basis Other operating income (expense)		(38)		40	(9)		101
Group Reporting Basis Total operating income (expense)		(39)		31	(12)		72
Expected credit losses / provision for credit losses		0		0	0		0
Group Reporting Basis Revenues		(39)		31	(12)		72
Operating Expenses		55		126	157		324
Group Reporting Basis Profit (Loss) Before Income Tax		(94)		(95)	(169)		(252)
Total Assets		1,611		1,565	1,611		1,565
Group Reporting Basis Total loans, net		0		0	0		0
Group Reporting Basis Goodwill		0		0	0		0
Group Reporting Basis Total deposits		$ 0		$ 0	$ 0		$ 0

[1] See Note 16, "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