ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Summary of Changes in Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Jan. 01, 2019
Allowance for Loan and Lease Losses [Roll Forward]
Cumulative effect of change in accounting principle						$ (331)		$ 17
Provision charged to income			$ 1,064	$ 182
Allowance for loan and lease losses
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Beginning balance	$ 2,171	$ 1,245	1,252	1,242
Cumulative effect of change in accounting principle					$ 0		$ 453
Allowance for loan and lease losses, beginning of period, adjusted					2,171		1,705
Charge-offs	(180)	(156)	(354)	(294)
Recoveries	33	50	70	99
Net charge-offs	(147)	(106)	(284)	(195)
Provision charged to income	424	88	1,027	180
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	2,448	1,227	2,448	1,227
Reserve for unfunded lending commitments
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Beginning balance	39	84	44	91
Cumulative effect of change in accounting principle					0		(2)
Allowance for loan and lease losses, beginning of period, adjusted					39		42
Provision for unfunded lending commitments	40	9	37	2
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	79	93	79	93
Commercial | Allowance for loan and lease losses
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Beginning balance	752	691	674	690
Cumulative effect of change in accounting principle					0		(176)
Allowance for loan and lease losses, beginning of period, adjusted					752		498
Charge-offs	(74)	(45)	(121)	(71)
Recoveries	3	12	6	14
Net charge-offs	(71)	(33)	(115)	(57)
Provision charged to income	554	22	852	47
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	1,235	680	1,235	680
Commercial | Reserve for unfunded lending commitments
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Beginning balance	38	84	44	91
Cumulative effect of change in accounting principle					0		(3)
Allowance for loan and lease losses, beginning of period, adjusted					38		41
Provision for unfunded lending commitments	31	9	28	2
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	69	93	69	93
Retail | Allowance for loan and lease losses
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Beginning balance	1,419	554	578	552
Cumulative effect of change in accounting principle					0		629
Allowance for loan and lease losses, beginning of period, adjusted					1,419		1,207
Charge-offs	(106)	(111)	(233)	(223)
Recoveries	30	38	64	85
Net charge-offs	(76)	(73)	(169)	(138)
Provision charged to income	(130)	66	175	133
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	1,213	547	1,213	547
Retail | Reserve for unfunded lending commitments
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Beginning balance	1	0	0	0
Cumulative effect of change in accounting principle					0		1
Allowance for loan and lease losses, beginning of period, adjusted					$ 1		$ 1
Provision for unfunded lending commitments	9	0	9	0
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	$ 10	$ 0	$ 10	$ 0