LOANS - Consumer Loans, Delinquencies and Non-Accrual Details (Details) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020 USD ($) category	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) category	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Financing Receivable, Past Due [Line Items]
Number of loan categories | category	2		2
Loans, net of unearned income	$ 666,911		$ 666,911		$ 699,483
Loans at fair value	5,524		5,524		4,085
Consumer
Financing Receivable, Past Due [Line Items]
Loans, current	274,700		274,700		302,803
Loans, net of unearned income	280,025		280,025		309,548
Non-accrual loans for which there are no loan loss reserves	88		88
Non-accrual loans for which there are loan loss reserves	1,602		1,602
Total non-accrual	1,690		1,690		1,816
90 days  past due and accruing	1,738		$ 1,738		$ 2,457
Loans less than this number of days past due are considered current			30 days		30 days
Loans at fair value	14		$ 14		$ 18
Number of days past due, non-accrual status			60 days
Unearned income	739		$ 739		783
Loans sold and/or reclassified to held-for-sale	386	$ 65	422	$ 2,355
Consumer | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Loans, past due	475		475		434
Consumer | 30 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans, past due	2,596		2,596		3,296
Consumer | 90 days past due
Financing Receivable, Past Due [Line Items]
Loans, past due	2,254		2,254		3,015
Consumer | In North America offices
Financing Receivable, Past Due [Line Items]
Loans, current	182,264		182,264		203,920
Loans, net of unearned income	186,169		186,169		209,093
Non-accrual loans for which there are no loan loss reserves	88		88
Non-accrual loans for which there are loan loss reserves	846		846
Total non-accrual	934		934		905
90 days  past due and accruing	1,431		1,431		2,215
Consumer | In North America offices | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Loans, past due	475		475		434
Consumer | In North America offices | 30 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans, past due	1,810		1,810		2,388
Consumer | In North America offices | 90 days past due
Financing Receivable, Past Due [Line Items]
Loans, past due	1,620		1,620		2,351
Consumer | In offices outside North America
Financing Receivable, Past Due [Line Items]
Loans, current	92,436		92,436		98,883
Loans, net of unearned income	93,856		93,856		100,455
Non-accrual loans for which there are no loan loss reserves	0		0
Non-accrual loans for which there are loan loss reserves	756		756
Total non-accrual	756		756		911
90 days  past due and accruing	307		307		242
Consumer | In offices outside North America | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Loans, past due	0		0		0
Consumer | In offices outside North America | 30 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans, past due	786		786		908
Consumer | In offices outside North America | 90 days past due
Financing Receivable, Past Due [Line Items]
Loans, past due	634		634		664
Consumer | Residential first mortgages
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	48,370		48,370		47,008
Loans at fair value	14		$ 14		$ 18
Number of days past due, non-accrual status			90 days		90 days
Consumer | Residential first mortgages | In North America offices
Financing Receivable, Past Due [Line Items]
Loans, current	47,152		$ 47,152		$ 45,942
Loans, net of unearned income	48,370		48,370		47,008
Non-accrual loans for which there are no loan loss reserves	84		84
Non-accrual loans for which there are loan loss reserves	466		466
Total non-accrual	550		550		479
90 days  past due and accruing	300		300		288
Mortgage loans in process of foreclosure	100		100		100
Consumer | Residential first mortgages | In North America offices | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Loans, past due	475		475		434
Consumer | Residential first mortgages | In North America offices | 30 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans, past due	458		458		411
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	200		200		100
Consumer | Residential first mortgages | In North America offices | 90 days past due
Financing Receivable, Past Due [Line Items]
Loans, past due	285		285		221
Consumer | Residential first mortgages | In North America offices | 90 days past due | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Loans, past due	300		300		300
Consumer | Residential first mortgages | In offices outside North America
Financing Receivable, Past Due [Line Items]
Loans, current	38,154		38,154		37,654
Loans, net of unearned income	38,507		38,507		38,024
Non-accrual loans for which there are no loan loss reserves	0		0
Non-accrual loans for which there are loan loss reserves	438		438
Total non-accrual	438		438		425
90 days  past due and accruing	0		0		0
Consumer | Residential first mortgages | In offices outside North America | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Loans, past due	0		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	184		184		210
Consumer | Residential first mortgages | In offices outside North America | 90 days past due
Financing Receivable, Past Due [Line Items]
Loans, past due	169		169		160
Consumer | Home equity loans
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	7,625		7,625		9,223
Consumer | Home equity loans | In North America offices
Financing Receivable, Past Due [Line Items]
Loans, current	7,347		7,347		8,860
Loans, net of unearned income	7,625		7,625		9,223
Non-accrual loans for which there are no loan loss reserves	2		2
Non-accrual loans for which there are loan loss reserves	359		359
Total non-accrual	361		361		405
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 | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Loans, past due	0		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	88		88		174
Consumer | Home equity loans | In North America offices | 90 days past due
Financing Receivable, Past Due [Line Items]
Loans, past due	190		190		189
Consumer | Credit cards
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	124,917		124,917		148,534
Consumer | Credit cards | In North America offices
Financing Receivable, Past Due [Line Items]
Loans, current	123,120		123,120		145,477
Loans, net of unearned income	125,485		125,485		149,163
Non-accrual loans for which there are no loan loss reserves	0		0
Non-accrual loans for which there are loan loss reserves	0		0
Total non-accrual	0		0		0
90 days  past due and accruing	1,131		1,131		1,927
Consumer | Credit cards | In North America offices | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Loans, past due	0		0		0
Consumer | Credit cards | In North America offices | 30 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans, past due	1,234		1,234		1,759
Consumer | Credit cards | In North America offices | 90 days past due
Financing Receivable, Past Due [Line Items]
Loans, past due	1,131		1,131		1,927
Consumer | Credit cards | In offices outside North America
Financing Receivable, Past Due [Line Items]
Loans, current	20,382		20,382		25,111
Loans, net of unearned income	21,108		21,108		25,909
Non-accrual loans for which there are no loan loss reserves	0		0
Non-accrual loans for which there are loan loss reserves	196		196
Total non-accrual	196		196		310
90 days  past due and accruing	307		307		242
Consumer | Credit cards | In offices outside North America | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Loans, past due	0		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	378		378		426
Consumer | Credit cards | In offices outside North America | 90 days past due
Financing Receivable, Past Due [Line Items]
Loans, past due	348		348		372
Consumer | Personal, small business and other
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	4,689		4,689		3,699
Consumer | Personal, small business and other | In North America offices
Financing Receivable, Past Due [Line Items]
Loans, current	4,645		4,645		3,641
Loans, net of unearned income	4,689		4,689		3,699
Non-accrual loans for which there are no loan loss reserves	2		2
Non-accrual loans for which there are loan loss reserves	21		21
Total non-accrual	23		23		21
90 days  past due and accruing	0		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		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	30		30		44
Consumer | Personal, small business and other | In North America offices | 90 days past due
Financing Receivable, Past Due [Line Items]
Loans, past due	14		14		14
Consumer | Personal, small business and other | In offices outside North America
Financing Receivable, Past Due [Line Items]
Loans, current	33,900		33,900		36,118
Loans, net of unearned income	34,241		34,241		36,522
Non-accrual loans for which there are no loan loss reserves	0		0
Non-accrual loans for which there are loan loss reserves	122		122
Total non-accrual	122		122		176
90 days  past due and accruing	0		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		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	224		224		272
Consumer | Personal, small business and other | In offices outside North America | 90 days past due
Financing Receivable, Past Due [Line Items]
Loans, past due	$ 117		$ 117		$ 132