LOANS - Consumer Loans, Delinquencies and Non-Accrual Details (Details) $ in Millions	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2020 USD ($) category	Mar. 31, 2020	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) category	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Financing Receivable, Past Due [Line Items]
Number of loan categories | category	2			2
Loans, net of unearned income	$ 685,292			$ 685,292		$ 699,483
Loans at fair value	5,799			5,799		4,085
Consumer
Financing Receivable, Past Due [Line Items]
Loans, current	275,143			275,143		302,803
Loans, net of unearned income	281,113			281,113		309,548
Non-accrual loans for which there are no loan loss reserves	207			207
Non-accrual loans for which there are loan loss reserves	1,622			1,622
Total non-accrual	1,829			1,829		1,816
90 days  past due and accruing	2,149			$ 2,149		$ 2,457
Loans less than this number of days past due are considered current				30 days		30 days
Loans at fair value	16			$ 16		$ 18
Number of days past due, non-accrual status				60 days
Unearned income	734			$ 734		783
Loans sold and/or reclassified to held-for-sale	12		$ 392	36	$ 2,295
Consumer | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Loans, past due	445			445		434
Consumer | 30 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans, past due	2,764			2,764		3,296
Consumer | 90 days past due
Financing Receivable, Past Due [Line Items]
Loans, past due	2,761			2,761		3,015
Consumer | In North America offices
Financing Receivable, Past Due [Line Items]
Loans, current	185,159			185,159		203,920
Loans, net of unearned income	189,582			189,582		209,093
Non-accrual loans for which there are no loan loss reserves	201			201
Non-accrual loans for which there are loan loss reserves	727			727
Total non-accrual	928			928		905
90 days  past due and accruing	1,877			1,877		2,215
Consumer | In North America offices | Government-guaranteed
Financing Receivable, Past Due [Line Items]
Loans, past due	445			445		434
Consumer | In North America offices | 30 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans, past due	1,906			1,906		2,388
Consumer | In North America offices | 90 days past due
Financing Receivable, Past Due [Line Items]
Loans, past due	2,072			2,072		2,351
Consumer | In offices outside North America
Financing Receivable, Past Due [Line Items]
Loans, current	89,984			89,984		98,883
Loans, net of unearned income	91,531			91,531		100,455
Non-accrual loans for which there are no loan loss reserves	6			6
Non-accrual loans for which there are loan loss reserves	895			895
Total non-accrual	901			901		911
90 days  past due and accruing	272			272		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	858			858		908
Consumer | In offices outside North America | 90 days past due
Financing Receivable, Past Due [Line Items]
Loans, past due	689			689		664
Consumer | Residential first mortgages
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	48,167			48,167		47,008
Loans at fair value	16			$ 16		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	46,923			$ 46,923		45,942
Loans, net of unearned income	48,167			48,167		47,008
Non-accrual loans for which there are no loan loss reserves	115			115
Non-accrual loans for which there are loan loss reserves	409			409
Total non-accrual	524			524		479
90 days  past due and accruing	282			282		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	445			445		434
Consumer | Residential first mortgages | In North America offices | 30 to 89 Days Past Due
Financing Receivable, Past Due [Line Items]
Loans, past due	541			541		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		100
Consumer | Residential first mortgages | In North America offices | 90 days past due
Financing Receivable, Past Due [Line Items]
Loans, past due	258			258		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	36,351			36,351		37,316
Loans, net of unearned income	36,745			36,745		37,686
Non-accrual loans for which there are no loan loss reserves	0			0
Non-accrual loans for which there are loan loss reserves	419			419
Total non-accrual	419			419		421
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	210			210		210
Consumer | Residential first mortgages | In offices outside North America | 90 days past due
Financing Receivable, Past Due [Line Items]
Loans, past due	184			184		160
Consumer | Home equity loans
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	8,524			8,524		9,223
Consumer | Home equity loans | In North America offices
Financing Receivable, Past Due [Line Items]
Loans, current	8,197			8,197		8,860
Loans, net of unearned income	8,524			8,524		9,223
Non-accrual loans for which there are no loan loss reserves	84			84
Non-accrual loans for which there are loan loss reserves	303			303
Total non-accrual	387			387		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	122			122		174
Consumer | Home equity loans | In North America offices | 90 days past due
Financing Receivable, Past Due [Line Items]
Loans, past due	205			205		189
Consumer | Credit cards
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	127,496			127,496		148,534
Consumer | Credit cards | In North America offices
Financing Receivable, Past Due [Line Items]
Loans, current	125,232			125,232		145,477
Loans, net of unearned income	128,032			128,032		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	1,595			1,595		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	1,205			1,205		1,759
Consumer | Credit cards | In North America offices | 90 days past due
Financing Receivable, Past Due [Line Items]
Loans, past due	1,595			1,595		1,927
Consumer | Credit cards | In offices outside North America
Financing Receivable, Past Due [Line Items]
Loans, current	20,212			20,212		25,111
Loans, net of unearned income	20,966			20,966		25,909
Non-accrual loans for which there are no loan loss reserves	5			5
Non-accrual loans for which there are loan loss reserves	265			265
Total non-accrual	270			270		310
90 days  past due and accruing	272			272		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	380			380		426
Consumer | Credit cards | In offices outside North America | 90 days past due
Financing Receivable, Past Due [Line Items]
Loans, past due	374			374		372
Consumer | Personal, small business and other
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	4,859			4,859		3,699
Consumer | Personal, small business and other | In North America offices
Financing Receivable, Past Due [Line Items]
Loans, current	4,807			4,807		3,641
Loans, net of unearned income	4,859			4,859		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	15			15
Total non-accrual	17			17		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	38			38		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	33,421			33,421		36,456
Loans, net of unearned income	33,820			33,820		36,860
Non-accrual loans for which there are no loan loss reserves	1			1
Non-accrual loans for which there are loan loss reserves	211			211
Total non-accrual	212			212		180
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	268			268		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	$ 131			$ 131		$ 132