LOANS - Consumer Loans, Delinquencies and Non-Accrual Details (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($) category	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) category	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Financing Receivable, Past Due [Line Items]
Number of loan categories | category	2		2
Total loans	$ 664,764		$ 664,764		$ 675,883
Loans held at fair value	7,159		7,159		6,854
Consumer
Financing Receivable, Past Due [Line Items]
Total loans	264,250		264,250		288,839
Non-accrual loans for which there is no ACLL	198		198		210
Non-accrual loans for which there is an ACLL	1,391		1,391		1,931
Total non-accrual	1,589		1,589		2,141
90 days  past due and accruing	1,222		1,222		2,038
Loans held at fair value	13		13		14
Unearned income	650		650		749
Loans sold and/or reclassified to held-for-sale	346	$ 386	1,178	$ 422
Consumer | Current
Financing Receivable, Past Due [Line Items]
Total loans	260,370		260,370		282,799
Consumer | Past Due | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Total loans	404		404		524
Consumer | 30 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans	1,767		1,767		2,696
Consumer | 90 days or more past due
Financing Receivable, Past Due [Line Items]
Total loans	1,709		1,709		2,820
Consumer | In North America offices
Financing Receivable, Past Due [Line Items]
Total loans	178,535		178,535		189,800
Non-accrual loans for which there is no ACLL	198		198		210
Non-accrual loans for which there is an ACLL	574		574		849
Total non-accrual	772		772		1,059
90 days  past due and accruing	1,050		1,050		1,662
Consumer | In North America offices | Current
Financing Receivable, Past Due [Line Items]
Total loans	175,711		175,711		185,599
Consumer | In North America offices | Past Due | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Total loans	404		404		524
Consumer | In North America offices | 30 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans	1,195		1,195		1,735
Consumer | In North America offices | 90 days or more past due
Financing Receivable, Past Due [Line Items]
Total loans	1,225		1,225		1,942
Consumer | In offices outside North America
Financing Receivable, Past Due [Line Items]
Total loans	85,715		85,715		99,039
Non-accrual loans for which there is no ACLL	0		0		0
Non-accrual loans for which there is an ACLL	817		817		1,082
Total non-accrual	817		817		1,082
90 days  past due and accruing	172		172		376
Consumer | In offices outside North America | Current
Financing Receivable, Past Due [Line Items]
Total loans	84,659		84,659		97,200
Consumer | In offices outside North America | Past Due | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Total loans	0		0		0
Consumer | In offices outside North America | 30 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans	572		572		961
Consumer | In offices outside North America | 90 days or more past due
Financing Receivable, Past Due [Line Items]
Total loans	484		484		878
Consumer | Residential first mortgages
Financing Receivable, Past Due [Line Items]
Total loans	44,345		44,345		47,778
Loans held at fair value	13		13		14
Consumer | Residential first mortgages | In North America offices
Financing Receivable, Past Due [Line Items]
Total loans	44,345		44,345		47,778
Non-accrual loans for which there is no ACLL	133		133		136
Non-accrual loans for which there is an ACLL	347		347		509
Total non-accrual	480		480		645
90 days  past due and accruing	267		267		332
Mortgage loans in process of foreclosure	100		100		100
Consumer | Residential first mortgages | In North America offices | Current
Financing Receivable, Past Due [Line Items]
Total loans	43,388		43,388		46,471
Consumer | Residential first mortgages | In North America offices | Past Due | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Total loans	393		393		524
Consumer | Residential first mortgages | In North America offices | 30 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans	289		289		402
Consumer | Residential first mortgages | In North America offices | 30 to 89 Days Past Due | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Total loans	100		100		200
Consumer | Residential first mortgages | In North America offices | 90 days or more past due
Financing Receivable, Past Due [Line Items]
Total loans	275		275		381
Consumer | Residential first mortgages | In North America offices | 90 days or more past due | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Total loans	300		300		300
Consumer | Residential first mortgages | In offices outside North America
Financing Receivable, Past Due [Line Items]
Total loans	34,339		34,339		39,969
Non-accrual loans for which there is no ACLL	0		0		0
Non-accrual loans for which there is an ACLL	419		419		486
Total non-accrual	419		419		486
90 days  past due and accruing	0		0		0
Consumer | Residential first mortgages | In offices outside North America | Current
Financing Receivable, Past Due [Line Items]
Total loans	33,993		33,993		39,557
Consumer | Residential first mortgages | In offices outside North America | Past Due | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Total loans	0		0		0
Consumer | Residential first mortgages | In offices outside North America | 30 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans	177		177		213
Consumer | Residential first mortgages | In offices outside North America | 90 days or more past due
Financing Receivable, Past Due [Line Items]
Total loans	169		169		199
Consumer | Home equity loans
Financing Receivable, Past Due [Line Items]
Total loans	5,485		5,485		7,128
Consumer | Home equity loans | In North America offices
Financing Receivable, Past Due [Line Items]
Total loans	5,485		5,485		7,128
Non-accrual loans for which there is no ACLL	65		65		72
Non-accrual loans for which there is an ACLL	210		210		307
Total non-accrual	275		275		379
90 days  past due and accruing	0		0		0
Home equity loans in process of foreclosure	100		100		100
Consumer | Home equity loans | In North America offices | Current
Financing Receivable, Past Due [Line Items]
Total loans	5,276		5,276		6,829
Consumer | Home equity loans | In North America offices | Past Due | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Total loans	0		0		0
Consumer | Home equity loans | In North America offices | 30 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans	48		48		78
Consumer | Home equity loans | In North America offices | 90 days or more past due
Financing Receivable, Past Due [Line Items]
Total loans	161		161		221
Consumer | Credit cards
Financing Receivable, Past Due [Line Items]
Total loans	125,001		125,001		129,813
Consumer | Credit cards | In North America offices
Financing Receivable, Past Due [Line Items]
Total loans	125,526		125,526		130,385
Non-accrual loans for which there is no ACLL	0		0		0
Non-accrual loans for which there is an ACLL	0		0		0
Total non-accrual	0		0		0
90 days  past due and accruing	783		783		1,330
Consumer | Credit cards | In North America offices | Current
Financing Receivable, Past Due [Line Items]
Total loans	123,897		123,897		127,827
Consumer | Credit cards | In North America offices | Past Due | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Total loans	0		0		0
Consumer | Credit cards | In North America offices | 30 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans	846		846		1,228
Consumer | Credit cards | In North America offices | 90 days or more past due
Financing Receivable, Past Due [Line Items]
Total loans	783		783		1,330
Consumer | Credit cards | In offices outside North America
Financing Receivable, Past Due [Line Items]
Total loans	17,763		17,763		22,692
Non-accrual loans for which there is no ACLL	0		0		0
Non-accrual loans for which there is an ACLL	187		187		384
Total non-accrual	187		187		384
90 days  past due and accruing	146		146		324
Consumer | Credit cards | In offices outside North America | Current
Financing Receivable, Past Due [Line Items]
Total loans	17,304		17,304		21,718
Consumer | Credit cards | In offices outside North America | Past Due | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Total loans	0		0		0
Consumer | Credit cards | In offices outside North America | 30 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans	233		233		429
Consumer | Credit cards | In offices outside North America | 90 days or more past due
Financing Receivable, Past Due [Line Items]
Total loans	226		226		545
Consumer | Personal, small business and other
Financing Receivable, Past Due [Line Items]
Total loans	3,179		3,179		4,509
Consumer | Personal, small business and other | In North America offices
Financing Receivable, Past Due [Line Items]
Total loans	3,179		3,179		4,509
Non-accrual loans for which there is no ACLL	0		0		2
Non-accrual loans for which there is an ACLL	17		17		33
Total non-accrual	17		17		35
90 days  past due and accruing	0		0		0
Consumer | Personal, small business and other | In North America offices | Current
Financing Receivable, Past Due [Line Items]
Total loans	3,150		3,150		4,472
Consumer | Personal, small business and other | In North America offices | Past Due | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Total loans	11		11		0
Consumer | Personal, small business and other | In North America offices | 30 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans	12		12		27
Consumer | Personal, small business and other | In North America offices | 90 days or more past due
Financing Receivable, Past Due [Line Items]
Total loans	6		6		10
Consumer | Personal, small business and other | In offices outside North America
Financing Receivable, Past Due [Line Items]
Total loans	33,613		33,613		36,378
Non-accrual loans for which there is no ACLL	0		0		0
Non-accrual loans for which there is an ACLL	211		211		212
Total non-accrual	211		211		212
90 days  past due and accruing	26		26		52
Consumer | Personal, small business and other | In offices outside North America | Current
Financing Receivable, Past Due [Line Items]
Total loans	33,362		33,362		35,925
Consumer | Personal, small business and other | In offices outside North America | Past Due | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Total loans	0		0		0
Consumer | Personal, small business and other | In offices outside North America | 30 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans	162		162		319
Consumer | Personal, small business and other | In offices outside North America | 90 days or more past due
Financing Receivable, Past Due [Line Items]
Total loans	$ 89		$ 89		$ 134