ALLOWANCE FOR CREDIT LOSSES ON LOANS AND LEASES - Changes in Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	$ 344.0
Provision for Credit Losses	18.0	$ 6.0
Balance at End of Period	371.0
Unfunded loan commitment
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	19.1	14.0
Charge- Offs	0.0	0.0
Recoveries	0.0	0.0
Net Charge- Offs	0.0	0.0
Provision for Credit Losses	(0.3)	0.0
Allowance for PCD Loans and Leases at Acquisition	0.0
Balance at End of Period	18.8	14.0
Total commercial loans and leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	261.2	280.0
Charge- Offs	(5.0)	(10.0)
Recoveries	2.9	4.0
Net Charge- Offs	(2.1)	(6.0)
Provision for Credit Losses	8.1	7.0
Allowance for PCD Loans and Leases at Acquisition	7.8
Balance at End of Period	275.0	281.0
Total commercial loans and leases | Commercial real estate
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	156.5	181.0
Charge- Offs	(1.0)	(1.0)
Recoveries	1.4	1.0
Net Charge- Offs	0.4	0.0
Provision for Credit Losses	4.4	3.0
Allowance for PCD Loans and Leases at Acquisition	4.4
Balance at End of Period	165.7	184.0
Total commercial loans and leases | Commercial and industrial
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	87.4	81.0
Charge- Offs	(3.2)	(8.0)
Recoveries	1.3	1.0
Net Charge- Offs	(1.9)	(7.0)
Provision for Credit Losses	2.8	5.0
Allowance for PCD Loans and Leases at Acquisition	3.4
Balance at End of Period	91.7	79.0
Total commercial loans and leases | Commercial leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	14.7	17.0
Charge- Offs	(0.1)	0.0
Recoveries	0.0	1.0
Net Charge- Offs	(0.1)	1.0
Provision for Credit Losses	(0.8)	(1.0)
Allowance for PCD Loans and Leases at Acquisition	0.0
Balance at End of Period	13.8	17.0
Total commercial loans and leases | Other
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	2.6	1.0
Charge- Offs	(0.7)	(1.0)
Recoveries	0.2	1.0
Net Charge- Offs	(0.5)	0.0
Provision for Credit Losses	1.7	0.0
Allowance for PCD Loans and Leases at Acquisition	0.0
Balance at End of Period	3.8	1.0
Total commercial loans and leases | Unfunded loan commitment
Allowance for Loan and Lease Losses [Roll Forward]
Provision for Credit Losses	0.0	0.0
Total consumer loans
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	83.1	83.0
Charge- Offs	(1.3)	(2.0)
Recoveries	1.5	1.0
Net Charge- Offs	0.2	(1.0)
Provision for Credit Losses	10.1	(1.0)
Allowance for PCD Loans and Leases at Acquisition	2.2
Balance at End of Period	95.6	81.0
Total consumer loans | Direct installment
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	26.4	26.0
Charge- Offs	0.0	0.0
Recoveries	0.2	0.0
Net Charge- Offs	0.2	0.0
Provision for Credit Losses	4.1	0.0
Allowance for PCD Loans and Leases at Acquisition	0.5
Balance at End of Period	31.2	26.0
Total consumer loans | Residential mortgages
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	33.1	34.0
Charge- Offs	(0.1)	0.0
Recoveries	0.2	0.0
Net Charge- Offs	0.1	0.0
Provision for Credit Losses	5.2	(2.0)
Allowance for PCD Loans and Leases at Acquisition	1.3
Balance at End of Period	39.7	32.0
Total consumer loans | Indirect installment
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	13.5	11.0
Charge- Offs	(1.0)	(2.0)
Recoveries	0.7	1.0
Net Charge- Offs	(0.3)	(1.0)
Provision for Credit Losses	1.0	1.0
Allowance for PCD Loans and Leases at Acquisition	0.0
Balance at End of Period	14.2	11.0
Total consumer loans | Consumer lines of credit
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	10.1	12.0
Charge- Offs	(0.2)	0.0
Recoveries	0.4	0.0
Net Charge- Offs	0.2	0.0
Provision for Credit Losses	(0.2)	0.0
Allowance for PCD Loans and Leases at Acquisition	0.4
Balance at End of Period	10.5	12.0
Total consumer loans | Unfunded loan commitment
Allowance for Loan and Lease Losses [Roll Forward]
Provision for Credit Losses	0.0	0.0
Allowance for credit losses on loans and leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	344.3	363.0
Charge- Offs	(6.3)	(12.0)
Recoveries	4.4	5.0
Net Charge- Offs	(1.9)	(7.1)
Provision for Credit Losses	18.2	6.0
Allowance for PCD Loans and Leases at Acquisition	10.0
Balance at End of Period	370.6	362.0
Allowance for credit losses on loans and leases and allowance for unfunded loan commitments
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	363.4	377.0
Charge- Offs	(6.3)	(12.0)
Recoveries	4.4	5.0
Net Charge- Offs	(1.9)	(7.0)
Provision for Credit Losses	17.9	6.0
Allowance for PCD Loans and Leases at Acquisition	10.0
Balance at End of Period	$ 389.4	$ 376.0