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
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
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			283	$ 241
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	$ 1,263		1,263
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	1,227	$ 1,253	1,242	1,236
Charge-offs	(159)	(127)	(453)	(363)
Recoveries	46	41	145	131
Net charge-offs	(113)	(86)	(308)	(232)
Provision charged to income	149	75	329	238
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	1,263	1,242	1,263	1,242
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	93	88	91	88
Provision for unfunded lending commitments	(48)	3	(46)	3
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	45	91	45	91
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	1,308	1,333	1,308	1,333
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	680	715	690	685
Charge-offs	(35)	(18)	(106)	(35)
Recoveries	3	2	17	10
Net charge-offs	(32)	(16)	(89)	(25)
Provision charged to income	64	8	111	47
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	712	707	712	707
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	93	88	91	88
Provision for unfunded lending commitments	(48)	3	(46)	3
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	45	91	45	91
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	757	798	757	798
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	547	538	552	551
Charge-offs	(124)	(109)	(347)	(328)
Recoveries	43	39	128	121
Net charge-offs	(81)	(70)	(219)	(207)
Provision charged to income	85	67	218	191
Allowance for loan and lease losses / Reserve for unfunded lending commitments, Ending balance	551	535	551	535
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
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
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	$ 551	$ 535	$ 551	$ 535

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.

+ References

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- Definition

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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- Definition

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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