ALLOWANCE FOR CREDIT LOSSES - Allowance for Credit Losses Roll Forward (Details) - USD ($) $ in Millions		3 Months Ended				6 Months Ended
	Jan. 01, 2020	Jun. 30, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020
Allowance for credit losses
Allowance for credit losses on loans (ACLL) at beginning of period	$ 12,783	$ 21,638	$ 26,298	$ 20,380	$ 12,783	$ 24,956	$ 12,783
Gross credit losses on loans		(1,844)		(2,528)		(4,052)	(5,007)
Gross recoveries on loans		524		367		984	787
Net credit losses on loans (NCLs)		(1,320)		(2,161)		(3,068)	(4,220)
Net reserve builds (releases)		(2,184)		5,195		(5,252)	9,289
Net specific reserve builds (releases) for loans		(262)		634		(421)	858
Total provision for credit losses on loans (PCLL)		(1,126)		7,990		(2,605)	14,367
Initial allowance for credit losses on newly purchased credit-deteriorated assets during the period		0		0		0	4
Other, net		46		89		(45)	(394)
ACLL at end of period		19,238		26,298	20,380	19,238	26,298
Allowance for credit losses on unfunded commitments
Allowance for credit losses on unfunded commitments (ACLUC) at beginning of period	1,456	2,012	1,859	1,813	1,456	2,655	1,456
Provision (release) for credit losses on unfunded lending commitments		44		113		(582)	670
Other, net		17		(67)		0	(73)
ACLUC at end of period		2,073		1,859	1,813	2,073	1,859
Total allowance for credit losses on loans, leases and unfunded lending commitments		21,311		28,157		21,311	28,157
Sales or transfers of various consumer loan portfolios to HFS
Sales or transfers of various consumer loan portfolios to HFS		0		(1)		0	(4)
FX translation		62		88		(46)	(395)
Other		(16)		2		1	5
Other, net		46		89		(45)	(394)
Variable post-charge-off third-party collection costs
Allowance for credit losses
Allowance for credit losses on loans (ACLL) at beginning of period	(443)	0		0	(443)	0	(443)
Total provision for credit losses on loans (PCLL)			$ 122	339	(18)
ACLL at end of period	(443)				0
Cumulative Effect, Period of Adoption, Adjustment
Allowance for credit losses
Allowance for credit losses on loans (ACLL) at beginning of period	4,201	0		0	4,201	0	4,201
Net reserve builds (releases)	4,900
ACLL at end of period	4,100				0
Allowance for credit losses on unfunded commitments
Allowance for credit losses on unfunded commitments (ACLUC) at beginning of period	(194)	0		0	(194)	0	(194)
ACLUC at end of period					0
Cumulative Effect, Period of Adoption, Adjusted Balance
Allowance for credit losses
Allowance for credit losses on loans (ACLL) at beginning of period	$ 16,541	$ 21,638		$ 20,380	16,541	$ 24,956	$ 16,541
ACLL at end of period					$ 20,380