LOANS - Consumer Loans, Delinquencies and Non-Accrual Details (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021 USD ($) category	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) category	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Financing Receivable, Past Due [Line Items]
Number of loan categories | category	2		2
Total loans	$ 676,834		$ 676,834		$ 675,883
Loans held at fair value	7,707		7,707		6,854
Consumer
Financing Receivable, Past Due [Line Items]
Total loans	275,895		275,895		288,839
Non-accrual loans for which there is no ACLL	201		201		210
Non-accrual loans for which there is an ACLL	1,605		1,605		1,931
Total non-accrual	1,806		1,806		2,141
90 days  past due and accruing	1,409		1,409		2,038
Loans held at fair value	14		14		14
Unearned income	676		676		749
Loans sold and/or reclassified to held-for-sale	95	$ 12	191	$ 36
Consumer | Current
Financing Receivable, Past Due [Line Items]
Total loans	271,572		271,572		282,799
Consumer | Past Due | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Total loans	402		402		524
Consumer | 30 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans	1,872		1,872		2,696
Consumer | 90 days or more past due
Financing Receivable, Past Due [Line Items]
Total loans	2,049		2,049		2,820
Consumer | In North America offices
Financing Receivable, Past Due [Line Items]
Total loans	179,502		179,502		189,800
Non-accrual loans for which there is no ACLL	201		201		210
Non-accrual loans for which there is an ACLL	678		678		849
Total non-accrual	879		879		1,059
90 days  past due and accruing	1,177		1,177		1,662
Consumer | In North America offices | Current
Financing Receivable, Past Due [Line Items]
Total loans	176,536		176,536		185,599
Consumer | In North America offices | Past Due | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Total loans	402		402		524
Consumer | In North America offices | 30 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans	1,149		1,149		1,735
Consumer | In North America offices | 90 days or more past due
Financing Receivable, Past Due [Line Items]
Total loans	1,415		1,415		1,942
Consumer | In offices outside North America
Financing Receivable, Past Due [Line Items]
Total loans	96,393		96,393		99,039
Non-accrual loans for which there is no ACLL	0		0		0
Non-accrual loans for which there is an ACLL	927		927		1,082
Total non-accrual	927		927		1,082
90 days  past due and accruing	232		232		376
Consumer | In offices outside North America | Current
Financing Receivable, Past Due [Line Items]
Total loans	95,036		95,036		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	723		723		961
Consumer | In offices outside North America | 90 days or more past due
Financing Receivable, Past Due [Line Items]
Total loans	634		634		878
Consumer | Residential first mortgages
Financing Receivable, Past Due [Line Items]
Total loans	44,835		44,835		47,778
Loans held at fair value	14		14		14
Consumer | Residential first mortgages | In North America offices
Financing Receivable, Past Due [Line Items]
Total loans	44,835		44,835		47,778
Non-accrual loans for which there is no ACLL	133		133		136
Non-accrual loans for which there is an ACLL	407		407		509
Total non-accrual	540		540		645
90 days  past due and accruing	257		257		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	43,811		43,811		46,471
Consumer | Residential first mortgages | In North America offices | Past Due | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Total loans	402		402		524
Consumer | Residential first mortgages | In North America offices | 30 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans	315		315		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	307		307		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	40,344		40,344		39,969
Non-accrual loans for which there is no ACLL	0		0		0
Non-accrual loans for which there is an ACLL	476		476		486
Total non-accrual	476		476		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	39,964		39,964		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	184		184		213
Consumer | Residential first mortgages | In offices outside North America | 90 days or more past due
Financing Receivable, Past Due [Line Items]
Total loans	196		196		199
Consumer | Home equity loans
Financing Receivable, Past Due [Line Items]
Total loans	6,168		6,168		7,128
Consumer | Home equity loans | In North America offices
Financing Receivable, Past Due [Line Items]
Total loans	6,168		6,168		7,128
Non-accrual loans for which there is no ACLL	68		68		72
Non-accrual loans for which there is an ACLL	253		253		307
Total non-accrual	321		321		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	5,937		5,937		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	51		51		78
Consumer | Home equity loans | In North America offices | 90 days or more past due
Financing Receivable, Past Due [Line Items]
Total loans	180		180		221
Consumer | Credit cards
Financing Receivable, Past Due [Line Items]
Total loans	124,295		124,295		129,813
Consumer | Credit cards | In North America offices
Financing Receivable, Past Due [Line Items]
Total loans	124,823		124,823		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	920		920		1,330
Consumer | Credit cards | In North America offices | Current
Financing Receivable, Past Due [Line Items]
Total loans	123,133		123,133		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	770		770		1,228
Consumer | Credit cards | In North America offices | 90 days or more past due
Financing Receivable, Past Due [Line Items]
Total loans	920		920		1,330
Consumer | Credit cards | In offices outside North America
Financing Receivable, Past Due [Line Items]
Total loans	20,776		20,776		22,692
Non-accrual loans for which there is no ACLL	0		0		0
Non-accrual loans for which there is an ACLL	233		233		384
Total non-accrual	233		233		384
90 days  past due and accruing	232		232		376
Consumer | Credit cards | In offices outside North America | Current
Financing Receivable, Past Due [Line Items]
Total loans	20,162		20,162		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	304		304		429
Consumer | Credit cards | In offices outside North America | 90 days or more past due
Financing Receivable, Past Due [Line Items]
Total loans	310		310		545
Consumer | Personal, small business and other
Financing Receivable, Past Due [Line Items]
Total loans	3,676		3,676		4,509
Consumer | Personal, small business and other | In North America offices
Financing Receivable, Past Due [Line Items]
Total loans	3,676		3,676		4,509
Non-accrual loans for which there is no ACLL	0		0		2
Non-accrual loans for which there is an ACLL	18		18		33
Total non-accrual	18		18		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	3,655		3,655		4,472
Consumer | Personal, small business and other | In North America offices | Past Due | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Total loans	0		0		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	13		13		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	8		8		10
Consumer | Personal, small business and other | In offices outside North America
Financing Receivable, Past Due [Line Items]
Total loans	35,273		35,273		36,378
Non-accrual loans for which there is no ACLL	0		0		0
Non-accrual loans for which there is an ACLL	218		218		212
Total non-accrual	218		218		212
90 days  past due and accruing	0		0		0
Consumer | Personal, small business and other | In offices outside North America | Current
Financing Receivable, Past Due [Line Items]
Total loans	34,910		34,910		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	235		235		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	$ 128		$ 128		$ 134