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, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period			$ 344.0
Provision for Credit Losses	$ 6.0	$ (1.0)	24.0	$ 5.0
Balance at End of Period	378.0		378.0
Unfunded loan commitment
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	18.8	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	(0.6)	0.0	(0.9)	0.0
Allowance for PCD Loans and Leases at Acquisition			0.0
Balance at End of Period	18.2	14.0	18.2	14.0
Total commercial loans and leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	275.0	281.0	261.2	280.0
Charge- Offs	(2.4)	(7.0)	(7.4)	(17.0)
Recoveries	3.7	3.0	6.6	7.0
Net (Charge- Offs) Recoveries	1.3	(4.0)	(0.8)	(10.0)
Provision for Credit Losses	(6.2)	(3.0)	1.9	4.0
Allowance for PCD Loans and Leases at Acquisition			7.8
Balance at End of Period	270.1	274.0	270.1	274.0
Total commercial loans and leases | Commercial real estate
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	165.7	184.0	156.5	181.0
Charge- Offs	(0.6)	(5.0)	(1.6)	(6.0)
Recoveries	0.7	2.0	2.1	3.0
Net (Charge- Offs) Recoveries	0.1	(3.0)	0.5	(3.0)
Provision for Credit Losses	(7.9)	(5.0)	(3.5)	(2.0)
Allowance for PCD Loans and Leases at Acquisition			4.4
Balance at End of Period	157.9	176.0	157.9	176.0
Total commercial loans and leases | Commercial and industrial
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	91.7	79.0	87.4	81.0
Charge- Offs	(1.1)	(1.0)	(4.3)	(9.0)
Recoveries	2.7	1.0	4.0	2.0
Net (Charge- Offs) Recoveries	1.6	0.0	(0.3)	(7.0)
Provision for Credit Losses	1.0	2.0	3.8	7.0
Allowance for PCD Loans and Leases at Acquisition			3.4
Balance at End of Period	94.3	81.0	94.3	81.0
Total commercial loans and leases | Commercial leases
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	13.8	17.0	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	(0.1)	(1.0)	(0.9)	(2.0)
Allowance for PCD Loans and Leases at Acquisition			0.0
Balance at End of Period	13.7	16.0	13.7	16.0
Total commercial loans and leases | Other
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	3.8	1.0	2.6	1.0
Charge- Offs	(0.7)	(1.0)	(1.4)	(2.0)
Recoveries	0.3	0.0	0.5	1.0
Net (Charge- Offs) Recoveries	(0.4)	(1.0)	(0.9)	(1.0)
Provision for Credit Losses	0.8	1.0	2.5	1.0
Allowance for PCD Loans and Leases at Acquisition			0.0
Balance at End of Period	4.2	1.0	4.2	1.0
Total commercial loans and leases | Unfunded loan commitment
Allowance for Loan and Lease Losses [Roll Forward]
Provision for Credit Losses	(1.0)	0.0	(1.0)	0.0
Total consumer loans
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	95.6	81.0	83.1	83.0
Charge- Offs	(1.8)	(1.0)	(3.1)	(3.0)
Recoveries	0.9	1.0	2.4	2.0
Net (Charge- Offs) Recoveries	(0.9)	0.0	(0.7)	(1.0)
Provision for Credit Losses	13.2	2.0	23.3	1.0
Allowance for PCD Loans and Leases at Acquisition			2.2
Balance at End of Period	107.9	83.0	107.9	83.0
Total consumer loans | Direct installment
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	31.2	26.0	26.4	26.0
Charge- Offs	(0.2)	0.0	(0.2)	0.0
Recoveries	0.2	0.0	0.4	0.0
Net (Charge- Offs) Recoveries	0.0	0.0	0.2	0.0
Provision for Credit Losses	3.0	1.0	7.1	1.0
Allowance for PCD Loans and Leases at Acquisition			0.5
Balance at End of Period	34.2	27.0	34.2	27.0
Total consumer loans | Residential mortgages
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	39.7	32.0	33.1	34.0
Charge- Offs	(0.2)	0.0	(0.3)	0.0
Recoveries	0.1	0.0	0.3	0.0
Net (Charge- Offs) Recoveries	(0.1)	0.0	0.0	0.0
Provision for Credit Losses	7.6	1.0	12.8	(1.0)
Allowance for PCD Loans and Leases at Acquisition			1.3
Balance at End of Period	47.2	33.0	47.2	33.0
Total consumer loans | Indirect installment
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	14.2	11.0	13.5	11.0
Charge- Offs	(1.2)	(1.0)	(2.2)	(3.0)
Recoveries	0.4	1.0	1.1	2.0
Net (Charge- Offs) Recoveries	(0.8)	0.0	(1.1)	(1.0)
Provision for Credit Losses	2.6	1.0	3.6	2.0
Allowance for PCD Loans and Leases at Acquisition			0.0
Balance at End of Period	16.0	12.0	16.0	12.0
Total consumer loans | Consumer lines of credit
Allowance for Loan and Lease Losses [Roll Forward]
Balance at Beginning of Period	10.5	12.0	10.1	12.0
Charge- Offs	(0.2)	0.0	(0.4)	0.0
Recoveries	0.2	0.0	0.6	0.0
Net (Charge- Offs) Recoveries	0.0	0.0	0.2	0.0
Provision for Credit Losses	0.0	(1.0)	(0.2)	(1.0)
Allowance for PCD Loans and Leases at Acquisition			0.4
Balance at End of Period	10.5	11.0	10.5	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	370.6	362.0	344.3	363.0
Charge- Offs	(4.2)	(8.0)	(10.5)	(20.0)
Recoveries	4.6	4.0	9.0	9.0
Net (Charge- Offs) Recoveries	0.4	(3.8)	(1.5)	(11.0)
Provision for Credit Losses	7.0	(1.0)	25.2	5.0
Allowance for PCD Loans and Leases at Acquisition			10.0
Balance at End of Period	378.0	357.0	378.0	357.0
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	389.4	376.0	363.4	377.0
Charge- Offs	(4.2)	(8.0)	(10.5)	(20.0)
Recoveries	4.6	4.0	9.0	9.0
Net (Charge- Offs) Recoveries	0.4	(4.0)	(1.5)	(11.0)
Provision for Credit Losses	6.4	(1.0)	24.3	5.0
Allowance for PCD Loans and Leases at Acquisition			10.0
Balance at End of Period	$ 396.2	$ 371.0	$ 396.2	$ 371.0