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, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period			$ 344.0
Provision for Credit Losses	$ 12.0	$ (2.0)	36.0	$ 3.0
Balance at End of Period	385.0		385.0
Unfunded loan commitment
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	18.2	14.0	19.1	14.0
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	1.1	4.0	0.2	4.0
Allowance for PCD Loans and Leases at Acquisition			0.0
Balance at End of Period	19.3	18.0	19.3	18.0
Total commercial loans and leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	270.1	274.0	261.2	280.0
Charge- Offs	(3.0)	(5.0)	(10.4)	(22.0)
Recoveries	1.7	4.0	8.3	11.0
Net (Charge- Offs) Recoveries	(1.3)	(1.0)	(2.1)	(11.0)
Provision for Credit Losses	1.1	(7.0)	3.0	(3.0)
Allowance for PCD Loans and Leases at Acquisition			7.8
Balance at End of Period	269.9	266.0	269.9	266.0
Total commercial loans and leases | Commercial real estate
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	157.9	176.0	156.5	181.0
Charge- Offs	(1.3)	(3.0)	(2.9)	(9.0)
Recoveries	0.5	2.0	2.6	5.0
Net (Charge- Offs) Recoveries	(0.8)	(1.0)	(0.3)	(4.0)
Provision for Credit Losses	(1.8)	(14.0)	(5.3)	(16.0)
Allowance for PCD Loans and Leases at Acquisition			4.4
Balance at End of Period	155.3	161.0	155.3	161.0
Total commercial loans and leases | Commercial and industrial
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	94.3	81.0	87.4	81.0
Charge- Offs	(0.8)	(2.0)	(5.1)	(11.0)
Recoveries	0.9	2.0	4.9	4.0
Net (Charge- Offs) Recoveries	0.1	0.0	(0.2)	(7.0)
Provision for Credit Losses	1.8	6.0	5.6	13.0
Allowance for PCD Loans and Leases at Acquisition			3.4
Balance at End of Period	96.2	87.0	96.2	87.0
Total commercial loans and leases | Commercial leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	13.7	16.0	14.7	17.0
Charge- Offs	0.0	0.0	(0.1)	0.0
Recoveries	0.0	0.0	0.0	1.0
Net (Charge- Offs) Recoveries	0.0	0.0	(0.1)	1.0
Provision for Credit Losses	0.5	0.0	(0.4)	(2.0)
Allowance for PCD Loans and Leases at Acquisition			0.0
Balance at End of Period	14.2	16.0	14.2	16.0
Total commercial loans and leases | Other
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	4.2	1.0	2.6	1.0
Charge- Offs	(0.9)	0.0	(2.3)	(2.0)
Recoveries	0.3	0.0	0.8	1.0
Net (Charge- Offs) Recoveries	(0.6)	0.0	(1.5)	(1.0)
Provision for Credit Losses	0.6	1.0	3.1	2.0
Allowance for PCD Loans and Leases at Acquisition			0.0
Balance at End of Period	4.2	2.0	4.2	2.0
Total commercial loans and leases | Unfunded loan commitment
Allowance for Loan and Lease Losses [Roll Forward]
Provision for Credit Losses	1.0	4.0	0.0	4.0
Total consumer loans
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	107.9	83.0	83.1	83.0
Charge- Offs	(2.7)	(2.0)	(5.8)	(5.0)
Recoveries	1.2	1.0	3.6	3.0
Net (Charge- Offs) Recoveries	(1.5)	(1.0)	(2.2)	(2.0)
Provision for Credit Losses	9.0	1.0	32.3	2.0
Allowance for PCD Loans and Leases at Acquisition			2.2
Balance at End of Period	115.4	83.0	115.4	83.0
Total consumer loans | Direct installment
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	34.2	27.0	26.4	26.0
Charge- Offs	(0.2)	(1.0)	(0.4)	(1.0)
Recoveries	0.1	0.0	0.5	0.0
Net (Charge- Offs) Recoveries	(0.1)	(1.0)	0.1	(1.0)
Provision for Credit Losses	2.0	0.0	9.1	1.0
Allowance for PCD Loans and Leases at Acquisition			0.5
Balance at End of Period	36.1	26.0	36.1	26.0
Total consumer loans | Residential mortgages
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	47.2	33.0	33.1	34.0
Charge- Offs	(0.3)	0.0	(0.6)	0.0
Recoveries	0.2	0.0	0.5	0.0
Net (Charge- Offs) Recoveries	(0.1)	0.0	(0.1)	0.0
Provision for Credit Losses	4.3	0.0	17.1	(1.0)
Allowance for PCD Loans and Leases at Acquisition			1.3
Balance at End of Period	51.4	33.0	51.4	33.0
Total consumer loans | Indirect installment
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	16.0	12.0	13.5	11.0
Charge- Offs	(1.9)	0.0	(4.1)	(3.0)
Recoveries	0.6	0.0	1.7	2.0
Net (Charge- Offs) Recoveries	(1.3)	0.0	(2.4)	(1.0)
Provision for Credit Losses	2.4	1.0	6.0	3.0
Allowance for PCD Loans and Leases at Acquisition			0.0
Balance at End of Period	17.1	13.0	17.1	13.0
Total consumer loans | Consumer lines of credit
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	10.5	11.0	10.1	12.0
Charge- Offs	(0.3)	(1.0)	(0.7)	(1.0)
Recoveries	0.3	1.0	0.9	1.0
Net (Charge- Offs) Recoveries	0.0	0.0	0.2	0.0
Provision for Credit Losses	0.3	0.0	0.1	(1.0)
Allowance for PCD Loans and Leases at Acquisition			0.4
Balance at End of Period	10.8	11.0	10.8	11.0
Total consumer loans | Unfunded loan commitment
Allowance for Loan and Lease Losses [Roll Forward]
Provision for Credit Losses	0.0	0.0	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	378.0	357.0	344.3	363.0
Charge- Offs	(5.7)	(7.0)	(16.2)	(27.0)
Recoveries	2.9	5.0	11.9	14.0
Net (Charge- Offs) Recoveries	(2.8)	(1.6)	(4.3)	(12.5)
Provision for Credit Losses	10.1	(6.0)	35.3	(1.0)
Allowance for PCD Loans and Leases at Acquisition			10.0
Balance at End of Period	385.3	349.0	385.3	349.0
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	396.2	371.0	363.4	377.0
Charge- Offs	(5.7)	(7.0)	(16.2)	(27.0)
Recoveries	2.9	5.0	11.9	14.0
Net (Charge- Offs) Recoveries	(2.8)	(2.0)	(4.3)	(13.0)
Provision for Credit Losses	11.2	(2.0)	35.5	3.0
Allowance for PCD Loans and Leases at Acquisition			10.0
Balance at End of Period	$ 404.6	$ 367.0	$ 404.6	$ 367.0