ALLOWANCE FOR CREDIT LOSSES ON LOANS AND LEASES - Changes in Allowance for Credit Losses (Detail) - USD ($) $ in Millions		3 Months Ended
	Jan. 01, 2020	Mar. 31, 2021	Mar. 31, 2020
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period		$ 363.0
Provision for Credit Losses		6.0	$ 48.0
Balance at End of Period		362.0
Unfunded Loan Commitment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	$ 3.0	14.0	3.0
Charge- Offs		0.0	0.0
Recoveries		0.0	0.0
Net Charge- Offs		0.0	0.0
Provision for Credit Losses		0.0	1.0
Other adjustments:		0.0	1.0
ASC 326 Adoption Impact	10.0
Initial ACL on PCD Loans			0.0
Balance at End of Period		14.0	14.0
Unfunded Loan Commitment [Member] | Other Non-interest Expense
Allowance for Loan and Lease Losses [Roll Forward]
Balance at End of Period			1.0
Commercial Real Estate [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	60.0	181.0	60.0
Charge- Offs		(1.0)	(2.0)
Recoveries		1.0	4.0
Net Charge- Offs		0.0	2.0
Provision for Credit Losses		3.0	12.0
ASC 326 Adoption Impact	38.0
Initial ACL on PCD Loans			40.0
Balance at End of Period		184.0	152.0
Commercial and Industrial [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	53.0	81.0	53.0
Charge- Offs		(8.0)	(4.0)
Recoveries		1.0	1.0
Net Charge- Offs		(7.0)	(3.0)
Provision for Credit Losses		5.0	26.0
ASC 326 Adoption Impact	8.0
Initial ACL on PCD Loans			4.0
Balance at End of Period		79.0	88.0
Commercial Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	11.0	17.0	11.0
Charge- Offs		0.0	0.0
Recoveries		1.0	0.0
Net Charge- Offs		1.0	0.0
Provision for Credit Losses		(1.0)	2.0
ASC 326 Adoption Impact	0.0
Initial ACL on PCD Loans			0.0
Balance at End of Period		17.0	13.0
Other [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	9.0	1.0	9.0
Charge- Offs		(1.0)	(1.0)
Recoveries		1.0	0.0
Net Charge- Offs		0.0	(1.0)
Provision for Credit Losses		0.0	2.0
ASC 326 Adoption Impact	(9.0)
Initial ACL on PCD Loans			0.0
Balance at End of Period		1.0	1.0
Total Commercial Loans and Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	133.0	280.0	133.0
Charge- Offs		(10.0)	(7.0)
Recoveries		4.0	5.0
Net Charge- Offs		(6.0)	(2.0)
Provision for Credit Losses		7.0	42.0
ASC 326 Adoption Impact	37.0
Initial ACL on PCD Loans			44.0
Balance at End of Period		281.0	254.0
Total Commercial Loans and Leases [Member] | Unfunded Loan Commitment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Provision for Credit Losses		0.0	0.0
ASC 326 Adoption Impact		0.0	8.0
Direct Installment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	13.0	26.0	13.0
Charge- Offs		0.0	(1.0)
Recoveries		0.0	0.0
Net Charge- Offs		0.0	(1.0)
Provision for Credit Losses		0.0	3.0
ASC 326 Adoption Impact	10.0
Initial ACL on PCD Loans			1.0
Balance at End of Period		26.0	26.0
Residential Mortgages [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	22.0	34.0	22.0
Charge- Offs		0.0	0.0
Recoveries		0.0	0.0
Net Charge- Offs		0.0	0.0
Provision for Credit Losses		(2.0)	(1.0)
ASC 326 Adoption Impact	6.0
Initial ACL on PCD Loans			4.0
Balance at End of Period		32.0	31.0
Indirect Installment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	19.0	11.0	19.0
Charge- Offs		(2.0)	(3.0)
Recoveries		1.0	1.0
Net Charge- Offs		(1.0)	(2.0)
Provision for Credit Losses		1.0	2.0
ASC 326 Adoption Impact	2.0
Initial ACL on PCD Loans			0.0
Balance at End of Period		11.0	21.0
Consumer Lines of Credit [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	9.0	12.0	9.0
Charge- Offs		0.0	(1.0)
Recoveries		0.0	0.0
Net Charge- Offs		0.0	(1.0)
Provision for Credit Losses		0.0	2.0
ASC 326 Adoption Impact	0.0
Initial ACL on PCD Loans			1.0
Balance at End of Period		12.0	11.0
Consumer Lines of Credit [Member] | Unfunded Loan Commitment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Provision for Credit Losses		0.0	0.0
ASC 326 Adoption Impact		0.0	2.0
Consumer Loan [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	63.0	83.0	63.0
Charge- Offs		(2.0)	(5.0)
Recoveries		1.0	1.0
Net Charge- Offs		(1.0)	(4.0)
Provision for Credit Losses		(1.0)	6.0
ASC 326 Adoption Impact	18.0
Initial ACL on PCD Loans			6.0
Balance at End of Period		81.0	89.0
Allowance For Credit Losses on Loans and Leases [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	196.0	363.0	196.0
Charge- Offs		(12.0)	(12.0)
Recoveries		5.0	6.0
Net Charge- Offs		(7.0)	(6.0)
Provision for Credit Losses		6.0	48.0
ASC 326 Adoption Impact	55.0
Initial ACL on PCD Loans			50.0
Balance at End of Period		362.0	343.0
Allowance For Credit Losses on Loans and Leases and Unfunded Loans Commitments [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	199.0	377.0	199.0
Charge- Offs		(12.0)	(12.0)
Recoveries		5.0	6.0
Net Charge- Offs		(7.0)	(6.0)
Provision for Credit Losses		6.0	49.0
ASC 326 Adoption Impact	$ 65.0
Initial ACL on PCD Loans			50.0
Balance at End of Period		$ 376.0	$ 357.0