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	12 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 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			163	$ 166
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	$ 1,253		1,253		$ 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	1,246	$ 1,224	1,236	1,236	1,236
Charge-offs	(120)	(128)	(236)	(261)
Recoveries	44	53	90	99
Net charge-offs	(76)	(75)	(146)	(162)
Provision charged to income	83	70	163	145
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	1,253	1,219	1,253	1,219	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	86	93	88	72	72
Provision for unfunded lending commitments	2	0	0	21
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	88	93	88	93	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	1,341	1,312	1,341	1,312
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	711	653	685	663	663
Charge-offs	(14)	(24)	(17)	(48)
Recoveries	2	10	8	15
Net charge-offs	(12)	(14)	(9)	(33)
Provision charged to income	16	(25)	39	(16)
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	715	614	715	614	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	86	93	88	72	72
Provision for unfunded lending commitments	2	0	0	21
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	88	93	88	93	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	803	707	803	707
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	535	571	551	573	573
Charge-offs	(106)	(104)	(219)	(213)
Recoveries	42	43	82	84
Net charge-offs	(64)	(61)	(137)	(129)
Provision charged to income	67	95	124	161
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	538	605	538	605	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	$ 538	$ 605	$ 538	$ 605

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

No definition available.

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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
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-Publisher FASB
-Name Accounting Standards Codification
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Amount of expense related to estimated loss from loan and lease transactions.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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