ALLOWANCE FOR CREDIT LOSSES - Allowance for Credit Losses Roll Forward (Details) - USD ($) $ in Millions		3 Months Ended				9 Months Ended
	Jan. 01, 2020	Sep. 30, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Allowance for credit losses
Allowance for credit losses on loans (ACLL) at beginning of period	$ 12,783	$ 19,238	$ 26,298		$ 12,783	$ 24,956	$ 12,783
Gross credit losses on loans		(1,389)	(2,367)			(5,441)	(7,374)
Gross recoveries on loans		428	448			1,412	1,235
Net credit losses on loans (NCLs)		(961)	(1,919)			(4,029)	(6,139)
Net reserve builds (releases)		(1,010)	164			(6,262)	9,453
Net specific reserve builds (releases) for loans		(139)	(152)			(560)	706
Total provision for credit losses on loans (PCLL)		(188)	1,931			(2,793)	16,298
Initial allowance for credit losses on newly purchased credit-deteriorated assets during the period		0	0			0	4
Other, net		(374)	116			(419)	(278)
ACLL at end of period		17,715	26,426	$ 26,298		17,715	26,426
Allowance for credit losses on unfunded commitments
Allowance for credit losses on unfunded commitments (ACLUC) at beginning of period	1,456	2,073	1,859		1,456	2,655	1,456
Provision (release) for credit losses on unfunded lending commitments		(13)	424			(595)	1,094
Other, net		3	16			3	(57)
ACLUC at end of period		2,063	2,299	1,859		2,063	2,299
Total allowance for credit losses on loans, leases and unfunded lending commitments		19,778	28,725			19,778	28,725
Sales or transfers of various consumer loan portfolios to HFS
Sales or transfers of various consumer loan portfolios to HFS		(278)	0			(278)	(4)
FX translation		(93)	116			(139)	(279)
Other		(3)	0			(2)	5
Other, net		(374)	116			(419)	(278)
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	$ 19,238	$ 26,298		$ 16,541	$ 24,956	$ 16,541
ACLL at end of period				$ 26,298