ALLOWANCE FOR CREDIT LOSSES ON LOANS AND LEASES - Changes in Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Allowance for Loan and Lease Losses [Roll Forward]
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest, Beginning Balance			$ 402.0
Charge- Offs			(69.6)
Provision for Credit Losses	$ 25.9	$ 11.2	58.5	$ 35.6
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest, Ending Balance	401.0		401.0
Unfunded loan commitment
Allowance for Loan and Lease Losses [Roll Forward]
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest, Beginning Balance	21.0	18.2	21.4	19.1
Charge- Offs	0.0	0.0	0.0	0.0
Recoveries	0.0	0.0	0.0	0.0
Net (Charge- Offs) Recoveries	0.0	0.0	0.0	0.0
Provision for Credit Losses	0.3	1.1	(0.1)	0.2
Allowance for PCD Loans and Leases at Acquisition				0.0
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest, Ending Balance	21.3	19.3	21.3	19.3
Total commercial loans and leases
Allowance for Loan and Lease Losses [Roll Forward]
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest, Beginning Balance	287.5	270.1	281.7	261.2
Charge- Offs	(38.7)	(3.0)	(60.7)	(10.4)
Recoveries	2.5	1.7	6.9	8.3
Net (Charge- Offs) Recoveries	(36.2)	(1.3)	(53.8)	(2.1)
Provision for Credit Losses	21.6	1.1	45.0	3.0
Allowance for PCD Loans and Leases at Acquisition				7.8
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest, Ending Balance	272.9	269.9	272.9	269.9
Total commercial loans and leases | Commercial real estate
Allowance for Loan and Lease Losses [Roll Forward]
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest, Beginning Balance	154.2	157.9	162.1	156.5
Charge- Offs	(0.6)	(1.3)	(8.9)	(2.9)
Recoveries	0.9	0.5	3.2	2.6
Net (Charge- Offs) Recoveries	0.3	(0.8)	(5.7)	(0.3)
Provision for Credit Losses	0.5	(1.8)	(1.4)	(5.3)
Allowance for PCD Loans and Leases at Acquisition				4.4
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest, Ending Balance	155.0	155.3	155.0	155.3
Total commercial loans and leases | Commercial and industrial
Allowance for Loan and Lease Losses [Roll Forward]
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest, Beginning Balance	114.3	94.3	102.1	87.4
Charge- Offs	(37.2)	(0.8)	(48.5)	(5.1)
Recoveries	1.4	0.9	3.0	4.9
Net (Charge- Offs) Recoveries	(35.8)	0.1	(45.5)	(0.2)
Provision for Credit Losses	17.1	1.8	39.0	5.6
Allowance for PCD Loans and Leases at Acquisition				3.4
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest, Ending Balance	95.6	96.2	95.6	96.2
Total commercial loans and leases | Commercial leases
Allowance for Loan and Lease Losses [Roll Forward]
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest, Beginning Balance	15.2	13.7	13.5	14.7
Charge- Offs	0.0	0.0	0.0	(0.1)
Recoveries	0.0	0.0	0.0	0.0
Net (Charge- Offs) Recoveries	0.0	0.0	0.0	(0.1)
Provision for Credit Losses	2.9	0.5	4.6	(0.4)
Allowance for PCD Loans and Leases at Acquisition				0.0
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest, Ending Balance	18.1	14.2	18.1	14.2
Total commercial loans and leases | Other
Allowance for Loan and Lease Losses [Roll Forward]
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest, Beginning Balance	3.8	4.2	4.0	2.6
Charge- Offs	(0.9)	(0.9)	(3.3)	(2.3)
Recoveries	0.2	0.3	0.7	0.8
Net (Charge- Offs) Recoveries	(0.7)	(0.6)	(2.6)	(1.5)
Provision for Credit Losses	1.1	0.6	2.8	3.1
Allowance for PCD Loans and Leases at Acquisition				0.0
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest, Ending Balance	4.2	4.2	4.2	4.2
Total commercial loans and leases | Unfunded loan commitment
Allowance for Loan and Lease Losses [Roll Forward]
Provision for Credit Losses	0.3	1.1	(0.1)	0.2
Total consumer loans
Allowance for Loan and Lease Losses [Roll Forward]
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest, Beginning Balance	125.2	107.9	120.0	83.1
Charge- Offs	(2.5)	(2.7)	(8.9)	(5.8)
Recoveries	1.0	1.2	3.1	3.6
Net (Charge- Offs) Recoveries	(1.5)	(1.5)	(5.8)	(2.2)
Provision for Credit Losses	4.0	9.0	13.5	32.3
Allowance for PCD Loans and Leases at Acquisition				2.2
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest, Ending Balance	127.7	115.4	127.7	115.4
Total consumer loans | Direct installment
Allowance for Loan and Lease Losses [Roll Forward]
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest, Beginning Balance	35.6	34.2	35.9	26.4
Charge- Offs	(0.1)	(0.2)	(0.5)	(0.4)
Recoveries	0.2	0.1	0.5	0.5
Net (Charge- Offs) Recoveries	0.1	(0.1)	0.0	0.1
Provision for Credit Losses	(1.0)	2.0	(1.2)	9.1
Allowance for PCD Loans and Leases at Acquisition				0.5
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest, Ending Balance	34.7	36.1	34.7	36.1
Total consumer loans | Residential mortgages
Allowance for Loan and Lease Losses [Roll Forward]
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest, Beginning Balance	63.4	47.2	55.5	33.1
Charge- Offs	(0.1)	(0.3)	(0.7)	(0.6)
Recoveries	0.1	0.2	0.3	0.5
Net (Charge- Offs) Recoveries	0.0	(0.1)	(0.4)	(0.1)
Provision for Credit Losses	4.0	4.3	12.3	17.1
Allowance for PCD Loans and Leases at Acquisition				1.3
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest, Ending Balance	67.4	51.4	67.4	51.4
Total consumer loans | Indirect installment
Allowance for Loan and Lease Losses [Roll Forward]
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest, Beginning Balance	16.6	16.0	17.3	13.5
Charge- Offs	(2.1)	(1.9)	(6.9)	(4.1)
Recoveries	0.5	0.6	1.6	1.7
Net (Charge- Offs) Recoveries	(1.6)	(1.3)	(5.3)	(2.4)
Provision for Credit Losses	1.1	2.4	4.1	6.0
Allowance for PCD Loans and Leases at Acquisition				0.0
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest, Ending Balance	16.1	17.1	16.1	17.1
Total consumer loans | Consumer lines of credit
Allowance for Loan and Lease Losses [Roll Forward]
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest, Beginning Balance	9.6	10.5	11.3	10.1
Charge- Offs	(0.2)	(0.3)	(0.8)	(0.7)
Recoveries	0.2	0.3	0.7	0.9
Net (Charge- Offs) Recoveries	0.0	0.0	(0.1)	0.2
Provision for Credit Losses	(0.1)	0.3	(1.7)	0.1
Allowance for PCD Loans and Leases at Acquisition				0.4
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest, Ending Balance	9.5	10.8	9.5	10.8
Allowance for credit losses on loans and leases
Allowance for Loan and Lease Losses [Roll Forward]
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest, Beginning Balance	412.7	378.0	401.7	344.3
Charge- Offs	(41.2)	(5.7)	(69.6)	(16.2)
Recoveries	3.5	2.9	10.0	11.9
Net (Charge- Offs) Recoveries	(37.7)	(2.8)	(59.6)	(4.3)
Provision for Credit Losses	25.6	10.1	58.5	35.3
Allowance for PCD Loans and Leases at Acquisition				10.0
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest, Ending Balance	400.6	385.3	400.6	385.3
Allowance for credit losses on loans and leases and allowance for unfunded loan commitments
Allowance for Loan and Lease Losses [Roll Forward]
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest, Beginning Balance	433.7	396.2	423.1	363.4
Charge- Offs	(41.2)	(5.7)	(69.6)	(16.2)
Recoveries	3.5	2.9	10.0	11.9
Net (Charge- Offs) Recoveries	(37.7)	(2.8)	(59.6)	(4.3)
Provision for Credit Losses	25.9	11.2	58.4	35.5
Allowance for PCD Loans and Leases at Acquisition				10.0
Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest, Ending Balance	$ 421.9	$ 404.6	$ 421.9	$ 404.6