LOANS - Consumer Loans, Delinquencies and Non-Accrual Details (Details) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2020 USD ($) category	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)
Financing Receivable, Past Due [Line Items]
Number of loan categories | category	2
Loans, net of unearned income	$ 721,020		$ 699,483
Loans at fair value	3,999		4,085
Consumer
Financing Receivable, Past Due [Line Items]
Loans, current	282,011		302,803
Loans, net of unearned income	288,430		309,548
Non-accrual loans for which there are no loan loss reserves	177
Non-accrual loans for which there are loan loss reserves	1,522
Total non-accrual	1,699		1,816
90 days past due and accruing	$ 2,359		$ 2,457
Loans less than this number of days past due are considered current	30 days		30 days
Loans at fair value	$ 18		$ 18
Number of days past due, non-accrual status	60 days
Unearned income	$ 771		783
Loans sold and/or reclassified to held-for-sale	0	$ 1,900
Consumer | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Loans, past due	413		434
Consumer | 30 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans, past due	3,122		3,296
Consumer | 90 days past due
Financing Receivable, Past Due [Line Items]
Loans, past due	2,884		3,015
Consumer | In North America offices
Financing Receivable, Past Due [Line Items]
Loans, current	192,260		203,920
Loans, net of unearned income	197,187		209,093
Non-accrual loans for which there are no loan loss reserves	170
Non-accrual loans for which there are loan loss reserves	756
Total non-accrual	926		905
90 days past due and accruing	2,123		2,215
Consumer | In North America offices | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Loans, past due	413		434
Consumer | In North America offices | 30 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans, past due	2,223		2,388
Consumer | In North America offices | 90 days past due
Financing Receivable, Past Due [Line Items]
Loans, past due	2,291		2,351
Consumer | In offices outside North America
Financing Receivable, Past Due [Line Items]
Loans, current	89,751		98,883
Loans, net of unearned income	91,243		100,455
Non-accrual loans for which there are no loan loss reserves	7
Non-accrual loans for which there are loan loss reserves	766
Total non-accrual	773		911
90 days past due and accruing	236		242
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	899		908
Consumer | In offices outside North America | 90 days past due
Financing Receivable, Past Due [Line Items]
Loans, past due	593		664
Consumer | Residential first mortgages
Financing Receivable, Past Due [Line Items]
Loans at fair value	$ 18		$ 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	$ 46,227		$ 45,942
Loans, net of unearned income	47,260		47,008
Non-accrual loans for which there are no loan loss reserves	142
Non-accrual loans for which there are loan loss reserves	358
Total non-accrual	500		479
90 days past due and accruing	274		288
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	413		434
Consumer | Residential first mortgages | In North America offices | 30 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans, past due	390		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	100		100
Consumer | Residential first mortgages | In North America offices | 90 days past due
Financing Receivable, Past Due [Line Items]
Loans, past due	230		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
Consumer | Residential first mortgages | In offices outside North America
Financing Receivable, Past Due [Line Items]
Loans, current	35,033		37,316
Loans, net of unearned income	35,400		37,686
Non-accrual loans for which there are no loan loss reserves	0
Non-accrual loans for which there are loan loss reserves	375
Total non-accrual	375		421
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	218		210
Consumer | Residential first mortgages | In offices outside North America | 90 days past due
Financing Receivable, Past Due [Line Items]
Loans, past due	149		160
Consumer | Home equity loans | In North America offices
Financing Receivable, Past Due [Line Items]
Loans, current	8,608		8,860
Loans, net of unearned income	8,936		9,223
Non-accrual loans for which there are no loan loss reserves	28
Non-accrual loans for which there are loan loss reserves	379
Total non-accrual	407		405
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	127		174
Consumer | Home equity loans | In North America offices | 90 days past due
Financing Receivable, Past Due [Line Items]
Loans, past due	201		189
Consumer | Credit cards | In North America offices
Financing Receivable, Past Due [Line Items]
Loans, current	133,794		145,477
Loans, net of unearned income	137,316		149,163
Non-accrual loans for which there are no loan loss reserves	0
Non-accrual loans for which there are loan loss reserves	0
Total non-accrual	0		0
90 days past due and accruing	1,849		1,927
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	1,673		1,759
Consumer | Credit cards | In North America offices | 90 days past due
Financing Receivable, Past Due [Line Items]
Loans, past due	1,849		1,927
Consumer | Credit cards | In offices outside North America
Financing Receivable, Past Due [Line Items]
Loans, current	21,073		25,111
Loans, net of unearned income	21,801		25,909
Non-accrual loans for which there are no loan loss reserves	0
Non-accrual loans for which there are loan loss reserves	243
Total non-accrual	243		310
90 days past due and accruing	236		242
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	403		426
Consumer | Credit cards | In offices outside North America | 90 days past due
Financing Receivable, Past Due [Line Items]
Loans, past due	325		372
Consumer | Personal, small business and other | In North America offices
Financing Receivable, Past Due [Line Items]
Loans, current	3,631		3,641
Loans, net of unearned income	3,675		3,699
Non-accrual loans for which there are no loan loss reserves	0
Non-accrual loans for which there are loan loss reserves	19
Total non-accrual	19		21
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	33		44
Consumer | Personal, small business and other | In North America offices | 90 days past due
Financing Receivable, Past Due [Line Items]
Loans, past due	11		14
Consumer | Personal, small business and other | In offices outside North America
Financing Receivable, Past Due [Line Items]
Loans, current	33,645		36,456
Loans, net of unearned income	34,042		36,860
Non-accrual loans for which there are no loan loss reserves	7
Non-accrual loans for which there are loan loss reserves	148
Total non-accrual	155		180
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	278		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	$ 119		$ 132