ALLOWANCE FOR CREDIT LOSSES - Allowance for Loan Losses Roll Forward (Details) - USD ($) $ in Millions		3 Months Ended
	Jan. 01, 2020	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020
Allowance for credit losses
Allowance for credit losses on loans (ACLL) at beginning of period	$ 12,783	$ 24,956		$ 20,380	$ 12,783
Gross credit losses on loans		(2,208)			(2,479)
Gross recoveries on loans		460			420
Net credit losses on loans (NCLs)		(1,748)			(2,059)
Net reserve builds (releases) for loans		(3,068)			4,094
Net specific reserve builds (releases) for loans		(159)			224
Total provision for credit losses on loans (PCLL)		(1,479)			6,377
Initial allowance for credit losses on newly purchased credit deteriorated assets during the period		0			4
Other, net		(91)			(483)
ACLL at end of period		21,638			20,380
Allowance for credit losses on unfunded commitments
Allowance for credit losses on unfunded commitments (ACLUC) at beginning of period	1,456	2,655		1,813	1,456
Provision (release) for credit losses on unfunded lending commitments		(626)			557
Other, net		(17)			(6)
ACLUC at end of period		2,012			1,813
Total allowance for credit losses on loans, leases and unfunded lending commitments		23,650			22,193
Sales or transfers of various consumer loan portfolios to HFS
Sales or transfers of various consumer loan portfolios to HFS		0			(3)
FX translation		(108)			(483)
Other		17			3
Revision of Prior Period, Change in Accounting Principle, Adjustment
Allowance for credit losses
Allowance for credit losses on loans (ACLL) at beginning of period	(443)	0			(443)
Total provision for credit losses on loans (PCLL)			$ 122	339	(18)
ACLL at end of period	(443)
Cumulative Effect, Period of Adoption, Adjustment
Allowance for credit losses
Allowance for credit losses on loans (ACLL) at beginning of period	4,201	0			4,201
Net reserve builds (releases) for loans	4,900
ACLL at end of period	4,100
Allowance for credit losses on unfunded commitments
Allowance for credit losses on unfunded commitments (ACLUC) at beginning of period	(194)	0			(194)
Cumulative Effect, Period of Adoption, Adjusted Balance
Allowance for credit losses
Allowance for credit losses on loans (ACLL) at beginning of period	16,541	24,956			16,541
Corporate
Allowance for credit losses
Allowance for credit losses on loans (ACLL) at beginning of period	2,886	5,402		3,451	2,886
Gross credit losses on loans		(203)			(139)
Gross recoveries on loans		17			12
Net credit losses on loans (NCLs)		(186)			(127)
Net reserve builds (releases) for loans		(1,273)			1,268
Net specific reserve builds (releases) for loans		(38)			48
Initial allowance for credit losses on newly purchased credit deteriorated assets during the period		0			0
Other, net		(7)			(30)
ACLL at end of period		4,084			3,451
Corporate | Revision of Prior Period, Change in Accounting Principle, Adjustment
Allowance for credit losses
Allowance for credit losses on loans (ACLL) at beginning of period	0	0			0
Corporate | Cumulative Effect, Period of Adoption, Adjustment
Allowance for credit losses
Allowance for credit losses on loans (ACLL) at beginning of period	(721)	0			(721)
Corporate | Cumulative Effect, Period of Adoption, Adjusted Balance
Allowance for credit losses
Allowance for credit losses on loans (ACLL) at beginning of period	2,165	5,402			2,165
Consumer
Allowance for credit losses
Allowance for credit losses on loans (ACLL) at beginning of period	9,897	19,554		$ 16,929	9,897
Gross credit losses on loans		(2,005)			(2,340)
Gross recoveries on loans		443			408
Net credit losses on loans (NCLs)		(1,562)			(1,932)
Net reserve builds (releases) for loans		(1,795)			2,826
Net specific reserve builds (releases) for loans		(121)			176
Initial allowance for credit losses on newly purchased credit deteriorated assets during the period		0			4
Other, net		(84)			(453)
ACLL at end of period		17,554			16,929
Consumer | Revision of Prior Period, Change in Accounting Principle, Adjustment
Allowance for credit losses
Allowance for credit losses on loans (ACLL) at beginning of period	(443)	0			(443)
Consumer | Cumulative Effect, Period of Adoption, Adjustment
Allowance for credit losses
Allowance for credit losses on loans (ACLL) at beginning of period	4,922	0			4,922
Consumer | Cumulative Effect, Period of Adoption, Adjusted Balance
Allowance for credit losses
Allowance for credit losses on loans (ACLL) at beginning of period	$ 14,376	$ 19,554			$ 14,376