ALLOWANCE FOR CREDIT LOSSES, NONPERFORMING ASSETS, AND CONCENTRATIONS OF CREDIT RISK - Summary of Changes in Allowance for Credit Losses (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Beginning balance	$ 1,242
Provision charged to income	393	$ 326	$ 321
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	1,252	1,242
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	1,242	1,236	1,236
Charge-offs	(615)	(494)	(512)
Recoveries	185	177	207
Net charge-offs	(430)	(317)	(305)
Provision charged to income	440	323	305
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	1,252	1,242	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	91	88	72
Provision for unfunded lending commitments	(47)	3	16
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	44	91	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, Beginning balance	1,333	1,324
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	1,296	1,333	1,324
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	690	685	663
Charge-offs	(140)	(52)	(75)
Recoveries	24	19	40
Net charge-offs	(116)	(33)	(35)
Provision charged to income	100	38	57
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	674	690	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	91	88	72
Provision for unfunded lending commitments	(47)	3	16
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	44	91	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, Beginning balance	781	773
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	718	781	773
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	552	551	573
Charge-offs	(475)	(442)	(437)
Recoveries	161	158	167
Net charge-offs	(314)	(284)	(270)
Provision charged to income	340	285	248
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	578	552	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	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, Beginning balance	552	551
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	$ 578	$ 552	551
Scenario, Adjustment \| Commercial \| Allowance for loan and lease losses
Allowance for Loan and Lease Losses [Roll Forward]
Provision charged to income			$ 50

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

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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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Amount of expense related to credit loss from transactions other than loan and lease transactions.

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