ALLOWANCE FOR CREDIT LOSSES ON LOANS AND LEASES - Schedule of Changes in Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period			$ 423.0
Charge- Offs			(64.5)		$ (81.8)
Provision for Credit Losses	$ 24.0	$ 23.0	67.1	$ 57.5
Balance at End of Period	437.0		437.0		423.0
Unfunded loan commitment
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	21.0	21.8	21.4	21.5	21.5
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	(0.9)	0.6	(1.3)	0.9
Balance at End of Period	20.1	22.4	20.1	22.4	21.4
Total commercial loans and leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	294.0	278.1	279.7	279.3	279.3
Charge- Offs	(19.8)	(25.2)	(55.2)	(46.2)
Recoveries	1.6	7.6	7.7	12.3
Net (Charge- Offs) Recoveries	(18.2)	(17.6)	(47.5)	(33.9)
Provision for Credit Losses	25.2	28.4	68.8	43.5
Balance at End of Period	301.0	288.9	301.0	288.9	279.7
Total commercial loans and leases | Commercial real estate
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	179.5	157.7	166.9	166.6	166.6
Charge- Offs	(9.2)	(13.8)	(19.8)	(24.1)	(38.6)
Recoveries	0.4	0.9	1.0	2.1
Net (Charge- Offs) Recoveries	(8.8)	(12.9)	(18.8)	(22.0)
Provision for Credit Losses	7.4	40.1	30.0	40.3
Balance at End of Period	178.1	184.9	178.1	184.9	166.9
Total commercial loans and leases | Commercial and industrial
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	88.3	93.5	85.6	87.8	87.8
Charge- Offs	(9.2)	(10.1)	(31.6)	(18.7)	(20.9)
Recoveries	1.0	6.5	6.0	9.0
Net (Charge- Offs) Recoveries	(8.2)	(3.6)	(25.6)	(9.7)
Provision for Credit Losses	14.9	(11.5)	35.0	0.3
Balance at End of Period	95.0	78.4	95.0	78.4	85.6
Total commercial loans and leases | Commercial leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	21.6	22.9	22.9	21.2	21.2
Charge- Offs	0.0	(0.1)	(0.2)	(0.3)	(0.3)
Recoveries	0.0	0.0	0.0	0.1
Net (Charge- Offs) Recoveries	0.0	(0.1)	(0.2)	(0.2)
Provision for Credit Losses	1.7	(1.2)	0.6	0.6
Balance at End of Period	23.3	21.6	23.3	21.6	22.9
Total commercial loans and leases | Other
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	4.6	4.0	4.3	3.7	3.7
Charge- Offs	(1.4)	(1.2)	(3.6)	(3.1)	(4.2)
Recoveries	0.2	0.2	0.7	1.1
Net (Charge- Offs) Recoveries	(1.2)	(1.0)	(2.9)	(2.0)
Provision for Credit Losses	1.2	1.0	3.2	2.3
Balance at End of Period	4.6	4.0	4.6	4.0	4.3
Total commercial loans and leases | Unfunded loan commitment
Allowance for Loan and Lease Losses [Roll Forward]
Provision for Credit Losses	(0.8)	0.6	(1.1)	1.0
Total consumer loans
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	138.1	140.7	143.1	126.3	126.3
Charge- Offs	(2.4)	(4.9)	(9.3)	(11.7)
Recoveries	0.9	1.0	2.8	3.5
Net (Charge- Offs) Recoveries	(1.5)	(3.9)	(6.5)	(8.2)
Provision for Credit Losses	(0.3)	(5.5)	(0.3)	13.2
Balance at End of Period	136.3	131.3	136.3	131.3	143.1
Total consumer loans | Direct installment
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	26.6	30.9	29.1	33.8	33.8
Charge- Offs	(0.1)	(0.2)	(0.7)	(0.8)	(1.8)
Recoveries	0.1	0.2	0.4	0.6
Net (Charge- Offs) Recoveries	0.0	0.0	(0.3)	(0.2)
Provision for Credit Losses	(0.6)	(1.7)	(2.8)	(4.4)
Balance at End of Period	26.0	29.2	26.0	29.2	29.1
Total consumer loans | Residential mortgages
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	95.4	88.2	95.9	70.5	70.5
Charge- Offs	(0.5)	(0.1)	(2.3)	(0.3)	(2.6)
Recoveries	0.1	0.1	0.3	0.2
Net (Charge- Offs) Recoveries	(0.4)	0.0	(2.0)	(0.1)
Provision for Credit Losses	(0.8)	(4.0)	0.3	13.8
Balance at End of Period	94.2	84.2	94.2	84.2	95.9
Total consumer loans | Indirect installment
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	9.3	13.1	9.5	12.8	12.8
Charge- Offs	(1.6)	(4.2)	(5.5)	(9.6)	(11.8)
Recoveries	0.6	0.6	1.8	2.1
Net (Charge- Offs) Recoveries	(1.0)	(3.6)	(3.7)	(7.5)
Provision for Credit Losses	0.8	(0.5)	3.3	3.7
Balance at End of Period	9.1	9.0	9.1	9.0	9.5
Total consumer loans | Consumer lines of credit
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	6.8	8.5	8.6	9.2	9.2
Charge- Offs	(0.2)	(0.4)	(0.8)	(1.0)	(1.6)
Recoveries	0.1	0.1	0.3	0.6
Net (Charge- Offs) Recoveries	(0.1)	(0.3)	(0.5)	(0.4)
Provision for Credit Losses	0.3	0.7	(1.1)	0.1
Balance at End of Period	7.0	8.9	7.0	8.9	8.6
Total consumer loans | Unfunded loan commitment
Allowance for Loan and Lease Losses [Roll Forward]
Provision for Credit Losses	(0.1)	0.0	(0.2)	(0.1)
Allowance for credit losses on loans and leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	432.1	418.8	422.8	405.6	405.6
Charge- Offs	(22.2)	(30.1)	(64.5)	(57.9)
Recoveries	2.5	8.6	10.5	15.8
Net (Charge- Offs) Recoveries	(19.7)	(21.5)	(54.0)	(42.1)
Provision for Credit Losses	24.9	22.9	68.5	56.7
Balance at End of Period	437.3	420.2	437.3	420.2	422.8
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	453.1	440.6	444.2	427.1	427.1
Charge- Offs	(22.2)	(30.1)	(64.5)	(57.9)
Recoveries	2.5	8.6	10.5	15.8
Net (Charge- Offs) Recoveries	(19.7)	(21.5)	(54.0)	(42.1)
Provision for Credit Losses	24.0	23.5	67.2	57.6
Balance at End of Period	$ 457.4	$ 442.6	$ 457.4	$ 442.6	$ 444.2