ALLOWANCE FOR CREDIT LOSSES - Allowance for Credit Losses Roll Forward by Segment (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
	Jan. 01, 2020	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Allowance for credit losses
Allowance for credit losses on loans (ACLL) at beginning of period	$ 12,783	$ 19,238	$ 26,298	$ 24,956	$ 12,783
Charge-offs		(1,389)	(2,367)	(5,441)	(7,374)
Recoveries		428	448	1,412	1,235
Replenishment of NCLs		961	1,919	4,029	6,139
Net reserve builds (releases)		(1,010)	164	(6,262)	9,453
Net specific reserve builds (releases)		(139)	(152)	(560)	706
Initial allowance for credit losses on newly purchased credit-deteriorated assets during the period		0	0	0	4
Other		(374)	116	(419)	(278)
ACLL at end of period		17,715	26,426	17,715	26,426
ACLL
Collectively evaluated		16,509		16,509		$ 23,094
Individually evaluated		1,206		1,206		1,860
Total ACLL		17,715	26,426	17,715	26,426	24,956
Loans, net of unearned income
Collectively evaluated		651,226		651,226		660,562
Individually evaluated		6,252		6,252		8,326
Loans held at fair value		7,159		7,159		6,854
Total loans, net of unearned income		664,764		664,764		675,883
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	0	(443)
ACLL at end of period	(443)
ACLL
Total ACLL	(443)					0
Purchased credit deteriorated
ACLL
Purchased credit deteriorated		0		0		2
Loans, net of unearned income
Purchased credit deteriorated		127		127		141
Corporate
Allowance for credit losses
Allowance for credit losses on loans (ACLL) at beginning of period	2,886	3,127	6,824	5,402	2,886
Charge-offs		(49)	(351)	(403)	(837)
Recoveries		10	26	89	61
Replenishment of NCLs		39	325	314	776
Net reserve builds (releases)		(44)	267	(2,137)	4,418
Net specific reserve builds (releases)		(21)	(161)	(189)	373
Initial allowance for credit losses on newly purchased credit-deteriorated assets during the period		0	0	0	0
Other		(15)	8	(29)	(18)
ACLL at end of period		3,047	6,938	3,047	6,938
ACLL
Collectively evaluated		2,732		2,732		4,887
Individually evaluated		315		315		515
Total ACLL		3,047	6,938	3,047	6,938	5,402
Loans, net of unearned income
Collectively evaluated		390,968		390,968		376,677
Individually evaluated		2,400		2,400		3,527
Loans held at fair value		7,146		7,146		6,840
Total loans, net of unearned income		400,514		400,514		387,044
Corporate | Variable post-charge-off third-party collection costs
Allowance for credit losses
Allowance for credit losses on loans (ACLL) at beginning of period	0			0	0
ACLL
Total ACLL						0
Corporate | Purchased credit deteriorated
ACLL
Purchased credit deteriorated		0		0		0
Loans, net of unearned income
Purchased credit deteriorated		0		0		0
Consumer
Allowance for credit losses
Allowance for credit losses on loans (ACLL) at beginning of period	9,897	16,111	19,474	19,554	9,897
Charge-offs		(1,340)	(2,016)	(5,038)	(6,537)
Recoveries		418	422	1,323	1,174
Replenishment of NCLs		922	1,594	3,715	5,363
Net reserve builds (releases)		(966)	(103)	(4,125)	5,035
Net specific reserve builds (releases)		(118)	9	(371)	333
Initial allowance for credit losses on newly purchased credit-deteriorated assets during the period		0	0	0	4
Other		(359)	108	(390)	(260)
ACLL at end of period		14,668	19,488	14,668	19,488
ACLL
Collectively evaluated		13,777		13,777		18,207
Individually evaluated		891		891		1,345
Total ACLL		14,668	19,488	14,668	19,488	19,554
Loans, net of unearned income
Collectively evaluated		260,258		260,258		283,885
Individually evaluated		3,852		3,852		4,799
Loans held at fair value		13		13		14
Total loans, net of unearned income		264,250		264,250		288,839
Consumer | 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	(443)
ACLL
Total ACLL						0
Consumer | Purchased credit deteriorated
ACLL
Purchased credit deteriorated		0		0		2
Loans, net of unearned income
Purchased credit deteriorated		127		127		141
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	0	4,201
Net reserve builds (releases)	4,900
ACLL at end of period	4,100
ACLL
Total ACLL	4,100					0
Cumulative Effect, Period of Adoption, Adjustment | Corporate
Allowance for credit losses
Allowance for credit losses on loans (ACLL) at beginning of period	(721)			0	(721)
ACLL
Total ACLL						0
Cumulative Effect, Period of Adoption, Adjustment | Consumer
Allowance for credit losses
Allowance for credit losses on loans (ACLL) at beginning of period	$ 4,922			$ 0	$ 4,922
ACLL
Total ACLL						$ 0