LOANS - Consumer Loans, Delinquencies and Non-Accrual Details (Details) $ in Millions	3 Months Ended
	Mar. 31, 2021 USD ($) category	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)
Financing Receivable, Past Due [Line Items]
Number of loan categories | category	2
Loans, net of unearned income	$ 665,988		$ 675,883
Loans at fair value	7,525		6,854
Consumer
Financing Receivable, Past Due [Line Items]
Loans, current	268,956		282,799
Loans, net of unearned income	274,034		288,839
Non-accrual loans for which there is no ACLL	199		210
Non-accrual loans for which there is an ACLL	1,785		1,931
Total non-accrual	1,984		2,141
90 days  past due and accruing	1,775		2,038
Loans at fair value	15		14
Unearned income	700		749
Loans sold and/or reclassified to held-for-sale	96	$ 24
Consumer | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Loans, past due	485		524
Consumer | 30 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans, past due	2,141		2,696
Consumer | 90 days or more past due
Financing Receivable, Past Due [Line Items]
Loans, past due	2,452		2,820
Consumer | In North America offices
Financing Receivable, Past Due [Line Items]
Loans, current	174,464		185,599
Loans, net of unearned income	178,025		189,800
Non-accrual loans for which there is no ACLL	199		210
Non-accrual loans for which there is an ACLL	762		849
Total non-accrual	961		1,059
90 days  past due and accruing	1,506		1,662
Consumer | In North America offices | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Loans, past due	485		524
Consumer | In North America offices | 30 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans, past due	1,355		1,735
Consumer | In North America offices | 90 days or more past due
Financing Receivable, Past Due [Line Items]
Loans, past due	1,721		1,942
Consumer | In offices outside North America
Financing Receivable, Past Due [Line Items]
Loans, current	94,492		97,200
Loans, net of unearned income	96,009		99,039
Non-accrual loans for which there is no ACLL	0		0
Non-accrual loans for which there is an ACLL	1,023		1,082
Total non-accrual	1,023		1,082
90 days  past due and accruing	269		376
Consumer | In offices outside North America | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Loans, past due	0		0
Consumer | In offices outside North America | 30 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans, past due	786		961
Consumer | In offices outside North America | 90 days or more past due
Financing Receivable, Past Due [Line Items]
Loans, past due	731		878
Consumer | Residential first mortgages
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	45,739		47,778
Loans at fair value	15		14
Consumer | Residential first mortgages | In North America offices
Financing Receivable, Past Due [Line Items]
Loans, current	44,638		46,471
Loans, net of unearned income	45,739		47,778
Non-accrual loans for which there is no ACLL	128		136
Non-accrual loans for which there is an ACLL	460		509
Total non-accrual	588		645
90 days  past due and accruing	325		332
Mortgage loans in process of foreclosure	100		100
Consumer | Residential first mortgages | In North America offices | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Loans, past due	485		524
Consumer | Residential first mortgages | In North America offices | 30 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans, past due	272		402
Consumer | Residential first mortgages | In North America offices | 30 to 89 Days Past Due | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Loans, past due	100		200
Consumer | Residential first mortgages | In North America offices | 90 days or more past due
Financing Receivable, Past Due [Line Items]
Loans, past due	344		381
Consumer | Residential first mortgages | In North America offices | 90 days or more past due | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Loans, past due	400		300
Consumer | Residential first mortgages | In offices outside North America
Financing Receivable, Past Due [Line Items]
Loans, current	39,426		39,557
Loans, net of unearned income	39,833		39,969
Non-accrual loans for which there is no ACLL	0		0
Non-accrual loans for which there is an ACLL	479		486
Total non-accrual	479		486
90 days  past due and accruing	0		0
Consumer | Residential first mortgages | In offices outside North America | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Loans, past due	0		0
Consumer | Residential first mortgages | In offices outside North America | 30 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans, past due	205		213
Consumer | Residential first mortgages | In offices outside North America | 90 days or more past due
Financing Receivable, Past Due [Line Items]
Loans, past due	202		199
Consumer | Home equity loans
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	6,638		7,128
Consumer | Home equity loans | In North America offices
Financing Receivable, Past Due [Line Items]
Loans, current	6,391		6,829
Loans, net of unearned income	6,638		7,128
Non-accrual loans for which there is no ACLL	69		72
Non-accrual loans for which there is an ACLL	268		307
Total non-accrual	337		379
90 days  past due and accruing	0		0
Home equity loans in process of foreclosure	100		100
Consumer | Home equity loans | In North America offices | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Loans, past due	0		0
Consumer | Home equity loans | In North America offices | 30 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans, past due	61		78
Consumer | Home equity loans | In North America offices | 90 days or more past due
Financing Receivable, Past Due [Line Items]
Loans, past due	186		221
Consumer | Credit cards
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	120,518		129,813
Consumer | Credit cards | In North America offices
Financing Receivable, Past Due [Line Items]
Loans, current	118,870		127,827
Loans, net of unearned income	121,048		130,385
Non-accrual loans for which there is no ACLL	0		0
Non-accrual loans for which there is an ACLL	0		0
Total non-accrual	0		0
90 days  past due and accruing	1,181		1,330
Consumer | Credit cards | In North America offices | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Loans, past due	0		0
Consumer | Credit cards | In North America offices | 30 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans, past due	997		1,228
Consumer | Credit cards | In North America offices | 90 days or more past due
Financing Receivable, Past Due [Line Items]
Loans, past due	1,181		1,330
Consumer | Credit cards | In offices outside North America
Financing Receivable, Past Due [Line Items]
Loans, current	20,397		21,718
Loans, net of unearned income	21,137		22,692
Non-accrual loans for which there is no ACLL	0		0
Non-accrual loans for which there is an ACLL	281		384
Total non-accrual	281		384
90 days  past due and accruing	269		376
Consumer | Credit cards | In offices outside North America | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Loans, past due	0		0
Consumer | Credit cards | In offices outside North America | 30 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans, past due	344		429
Consumer | Credit cards | In offices outside North America | 90 days or more past due
Financing Receivable, Past Due [Line Items]
Loans, past due	396		545
Consumer | Personal, small business and other
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	4,600		4,509
Consumer | Personal, small business and other | In North America offices
Financing Receivable, Past Due [Line Items]
Loans, current	4,565		4,472
Loans, net of unearned income	4,600		4,509
Non-accrual loans for which there is no ACLL	2		2
Non-accrual loans for which there is an ACLL	34		33
Total non-accrual	36		35
90 days  past due and accruing	0		0
Consumer | Personal, small business and other | In North America offices | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Loans, past due	0		0
Consumer | Personal, small business and other | In North America offices | 30 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans, past due	25		27
Consumer | Personal, small business and other | In North America offices | 90 days or more past due
Financing Receivable, Past Due [Line Items]
Loans, past due	10		10
Consumer | Personal, small business and other | In offices outside North America
Financing Receivable, Past Due [Line Items]
Loans, current	34,669		35,925
Loans, net of unearned income	35,039		36,378
Non-accrual loans for which there is no ACLL	0		0
Non-accrual loans for which there is an ACLL	263		212
Total non-accrual	263		212
90 days  past due and accruing	0		0
Consumer | Personal, small business and other | In offices outside North America | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Loans, past due	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]
Loans, past due	237		319
Consumer | Personal, small business and other | In offices outside North America | 90 days or more past due
Financing Receivable, Past Due [Line Items]
Loans, past due	$ 133		$ 134