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	9 Months Ended	12 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Beginning balance			$ 1,236
Provision charged to income			241	$ 238
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	$ 1,242		1,242		$ 1,236
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	1,253	$ 1,219	1,236	1,236	1,236
Charge-offs	(127)	(120)	(363)	(381)
Recoveries	41	55	131	154
Net charge-offs	(86)	(65)	(232)	(227)
Provision charged to income	75	70	238	215
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	1,242	1,224	1,242	1,224	1,236
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	88	93	88	72	72
Provision for unfunded lending commitments	3	2	3	23
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	91	95	91	95	88
Allowance for loan and lease losses and reserve for off-balance sheet activities, total
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	1,333	1,319	1,333	1,319
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	715	614	685	663	663
Charge-offs	(18)	(12)	(35)	(60)
Recoveries	2	12	10	27
Net charge-offs	(16)	0	(25)	(33)
Provision charged to income	8	24	47	8
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	707	638	707	638	685
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	88	93	88	72	72
Provision for unfunded lending commitments	3	2	3	23
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	91	95	91	95	88
Commercial \| Allowance for loan and lease losses and reserve for off-balance sheet activities, total
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	798	733	798	733
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	538	605	551	573	573
Charge-offs	(109)	(108)	(328)	(321)
Recoveries	39	43	121	127
Net charge-offs	(70)	(65)	(207)	(194)
Provision charged to income	67	46	191	207
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	535	586	535	586	551
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	0	0	0	0	0
Provision for unfunded lending commitments	0	0	0	0
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	0	0	0	0	$ 0
Retail \| Allowance for loan and lease losses and reserve for off-balance sheet activities, total
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	$ 535	$ 586	$ 535	$ 586

Amount of provision (credit) for unfunded lending commitments recorded during the reporting period.

No definition available.

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Amount of recovery of loans and lease receivables which had previously been fully or partially written-off as bad debts.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss.

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Amount, after recoveries, of loans and leases that have been written-off.

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Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

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Amount of expense related to estimated loss from loan and lease transactions.

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