ALLOWANCE FOR CREDIT LOSSES ON LOANS AND LEASES - Changes in Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period			$ 402.0
Charge- Offs			(28.4)
Provision for Credit Losses	$ 18.5	$ 6.4	32.6	$ 24.4
Balance at End of Period	413.0		413.0
Unfunded loan commitment
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	20.5	18.8	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	0.5	(0.6)	(0.4)	(0.9)
Allowance for PCD Loans and Leases at Acquisition				0.0
Balance at End of Period	21.0	18.2	21.0	18.2
Total commercial loans and leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	279.7	275.0	281.7	261.2
Charge- Offs	(8.9)	(2.4)	(22.0)	(7.4)
Recoveries	2.2	3.7	4.4	6.6
Net (Charge- Offs) Recoveries	(6.7)	1.3	(17.6)	(0.8)
Provision for Credit Losses	14.5	(6.2)	23.4	1.9
Allowance for PCD Loans and Leases at Acquisition				7.8
Balance at End of Period	287.5	270.1	287.5	270.1
Total commercial loans and leases | Commercial real estate
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	159.2	165.7	162.1	156.5
Charge- Offs	(1.8)	(0.6)	(8.3)	(1.6)
Recoveries	1.3	0.7	2.3	2.1
Net (Charge- Offs) Recoveries	(0.5)	0.1	(6.0)	0.5
Provision for Credit Losses	(4.5)	(7.9)	(1.9)	(3.5)
Allowance for PCD Loans and Leases at Acquisition				4.4
Balance at End of Period	154.2	157.9	154.2	157.9
Total commercial loans and leases | Commercial and industrial
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	101.7	91.7	102.1	87.4
Charge- Offs	(5.5)	(1.1)	(11.3)	(4.3)
Recoveries	0.7	2.7	1.6	4.0
Net (Charge- Offs) Recoveries	(4.8)	1.6	(9.7)	(0.3)
Provision for Credit Losses	17.4	1.0	21.9	3.8
Allowance for PCD Loans and Leases at Acquisition				3.4
Balance at End of Period	114.3	94.3	114.3	94.3
Total commercial loans and leases | Commercial leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	14.8	13.8	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	0.4	(0.1)	1.7	(0.9)
Allowance for PCD Loans and Leases at Acquisition				0.0
Balance at End of Period	15.2	13.7	15.2	13.7
Total commercial loans and leases | Other
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	4.0	3.8	4.0	2.6
Charge- Offs	(1.6)	(0.7)	(2.4)	(1.4)
Recoveries	0.2	0.3	0.5	0.5
Net (Charge- Offs) Recoveries	(1.4)	(0.4)	(1.9)	(0.9)
Provision for Credit Losses	1.2	0.8	1.7	2.5
Allowance for PCD Loans and Leases at Acquisition				0.0
Balance at End of Period	3.8	4.2	3.8	4.2
Total commercial loans and leases | Unfunded loan commitment
Allowance for Loan and Lease Losses [Roll Forward]
Provision for Credit Losses	0.5	(0.6)	(0.4)	(0.9)
Total consumer loans
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	123.7	95.6	120.0	83.1
Charge- Offs	(2.8)	(1.8)	(6.4)	(3.1)
Recoveries	0.8	0.9	2.1	2.4
Net (Charge- Offs) Recoveries	(2.0)	(0.9)	(4.3)	(0.7)
Provision for Credit Losses	3.5	13.2	9.5	23.3
Allowance for PCD Loans and Leases at Acquisition				2.2
Balance at End of Period	125.2	107.9	125.2	107.9
Total consumer loans | Direct installment
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	36.2	31.2	35.9	26.4
Charge- Offs	(0.1)	(0.2)	(0.4)	(0.2)
Recoveries	0.1	0.2	0.3	0.4
Net (Charge- Offs) Recoveries	0.0	0.0	(0.1)	0.2
Provision for Credit Losses	(0.6)	3.0	(0.2)	7.1
Allowance for PCD Loans and Leases at Acquisition				0.5
Balance at End of Period	35.6	34.2	35.6	34.2
Total consumer loans | Residential mortgages
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	60.4	39.7	55.5	33.1
Charge- Offs	(0.2)	(0.2)	(0.6)	(0.3)
Recoveries	0.0	0.1	0.2	0.3
Net (Charge- Offs) Recoveries	(0.2)	(0.1)	(0.4)	0.0
Provision for Credit Losses	3.2	7.6	8.3	12.8
Allowance for PCD Loans and Leases at Acquisition				1.3
Balance at End of Period	63.4	47.2	63.4	47.2
Total consumer loans | Indirect installment
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	16.6	14.2	17.3	13.5
Charge- Offs	(2.2)	(1.2)	(4.8)	(2.2)
Recoveries	0.5	0.4	1.1	1.1
Net (Charge- Offs) Recoveries	(1.7)	(0.8)	(3.7)	(1.1)
Provision for Credit Losses	1.7	2.6	3.0	3.6
Allowance for PCD Loans and Leases at Acquisition				0.0
Balance at End of Period	16.6	16.0	16.6	16.0
Total consumer loans | Consumer lines of credit
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	10.5	10.5	11.3	10.1
Charge- Offs	(0.3)	(0.2)	(0.6)	(0.4)
Recoveries	0.2	0.2	0.5	0.6
Net (Charge- Offs) Recoveries	(0.1)	0.0	(0.1)	0.2
Provision for Credit Losses	(0.8)	0.0	(1.6)	(0.2)
Allowance for PCD Loans and Leases at Acquisition				0.4
Balance at End of Period	9.6	10.5	9.6	10.5
Allowance for credit losses on loans and leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	403.4	370.6	401.7	344.3
Charge- Offs	(11.7)	(4.2)	(28.4)	(10.5)
Recoveries	3.0	4.6	6.5	9.0
Net (Charge- Offs) Recoveries	(8.7)	0.4	(21.9)	(1.5)
Provision for Credit Losses	18.0	7.0	32.9	25.2
Allowance for PCD Loans and Leases at Acquisition				10.0
Balance at End of Period	412.7	378.0	412.7	378.0
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	423.9	389.4	423.1	363.4
Charge- Offs	(11.7)	(4.2)	(28.4)	(10.5)
Recoveries	3.0	4.6	6.5	9.0
Net (Charge- Offs) Recoveries	(8.7)	0.4	(21.9)	(1.5)
Provision for Credit Losses	18.5	6.4	32.5	24.3
Allowance for PCD Loans and Leases at Acquisition				10.0
Balance at End of Period	$ 433.7	$ 396.2	$ 433.7	$ 396.2