LOANS - Corporate Troubled Debt Restructurings (Details) - Corporate - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Financing receivable impaired
Financing Receivable, Troubled Debt Restructuring, Postmodification	$ 285	$ 305
Period within which default occurred post-modification	1 year
Number of days past due, default status	60 days
TDR balances	$ 450	549
TDR in payment default	$ 0	35
Commercial banking
Financing receivable impaired
Number of days past due, default status	90 days
Commercial and industrial
Financing receivable impaired
Financing Receivable, Troubled Debt Restructuring, Postmodification	$ 247	283
TDR balances	325	426
TDR in payment default	0	35
Financial institutions
Financing receivable impaired
TDR balances	0	0
TDR in payment default	0	0
Mortgage and real estate
Financing receivable impaired
Financing Receivable, Troubled Debt Restructuring, Postmodification	19	16
TDR balances	92	79
TDR in payment default	0	0
Lease financing
Financing receivable impaired
TDR balances	0	0
TDR in payment default	0	0
Other
Financing receivable impaired
Financing Receivable, Troubled Debt Restructuring, Postmodification	19	6
TDR balances	33	44
TDR in payment default	0	0
TDRs involving changes in the amount and/or timing of principal payments
Financing receivable impaired
Financing Receivable, Troubled Debt Restructuring, Postmodification	6	25
TDRs involving changes in the amount and/or timing of principal payments | Commercial and industrial
Financing receivable impaired
Financing Receivable, Troubled Debt Restructuring, Postmodification	0	19
TDRs involving changes in the amount and/or timing of principal payments | Mortgage and real estate
Financing receivable impaired
Financing Receivable, Troubled Debt Restructuring, Postmodification	0	0
TDRs involving changes in the amount and/or timing of principal payments | Other
Financing receivable impaired
Financing Receivable, Troubled Debt Restructuring, Postmodification	6	6
TDRs involving vhanges in the amount and/or timing of interest payments
Financing receivable impaired
Financing Receivable, Troubled Debt Restructuring, Postmodification	0	0
TDRs involving vhanges in the amount and/or timing of interest payments | Commercial and industrial
Financing receivable impaired
Financing Receivable, Troubled Debt Restructuring, Postmodification	0	0
TDRs involving vhanges in the amount and/or timing of interest payments | Mortgage and real estate
Financing receivable impaired
Financing Receivable, Troubled Debt Restructuring, Postmodification	0	0
TDRs involving vhanges in the amount and/or timing of interest payments | Other
Financing receivable impaired
Financing Receivable, Troubled Debt Restructuring, Postmodification	0	0
TDRs invovling changes in the amount and/or timing of both principal and interest payments
Financing receivable impaired
Financing Receivable, Troubled Debt Restructuring, Postmodification	279	280
TDRs invovling changes in the amount and/or timing of both principal and interest payments | Commercial and industrial
Financing receivable impaired
Financing Receivable, Troubled Debt Restructuring, Postmodification	247	264
TDRs invovling changes in the amount and/or timing of both principal and interest payments | Mortgage and real estate
Financing receivable impaired
Financing Receivable, Troubled Debt Restructuring, Postmodification	19	16
TDRs invovling changes in the amount and/or timing of both principal and interest payments | Other
Financing receivable impaired
Financing Receivable, Troubled Debt Restructuring, Postmodification	$ 13	$ 0