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	12 Months Ended
Mar. 31, 2018	Mar. 31, 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	78	$ 96
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	1,246		$ 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	1,236	1,236	1,236
Charge-offs	(116)	(133)
Recoveries	46	46
Net recoveries (charge-offs)	(70)	(87)
Provision charged to income	80	75
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	1,246	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	72	72
Provision for unfunded lending commitments	(2)	21
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	86	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	1,332	1,317
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	685	663	663
Charge-offs	(3)	(24)
Recoveries	6	5
Net recoveries (charge-offs)	3	(19)
Provision charged to income	23	9
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	711	653	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	72	72
Provision for unfunded lending commitments	(2)	21
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	86	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	797	746
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	551	573	573
Charge-offs	(113)	(109)
Recoveries	40	41
Net recoveries (charge-offs)	(73)	(68)
Provision charged to income	57	66
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	535	571	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
Provision for unfunded lending commitments	0	0
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	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	$ 571

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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Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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- Definition

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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