Allowance for Credit Losses (Changes in Allowance for Credit Losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
LOANS AND LEASES
Balance at Beginning of Period	$ 138.4	$ 190.7	$ 104.5
Charge-Offs	(6.0)	(0.7)	(9.7)
Recoveries	10.2	7.0	6.5
Net Recoveries (Charge-Offs)	4.2	6.3	(3.2)
Provision for Credit Losses	1.7	(58.6)	91.6
Balance at End of Period	144.3	138.4	190.7
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	34.1	61.1	19.9
Charge-Offs	0.0	0.0	0.0
Recoveries	0.0	0.0	0.0
Net Recoveries (Charge-Offs)	0.0	0.0	0.0
Provision for Credit Losses	4.4	(27.0)	32.3
Balance at End of Period	38.5	34.1	61.1
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	11.2	7.3	0.0
Charge-Offs	0.0	0.0	0.0
Recoveries	0.0	0.0	0.0
Net Recoveries (Charge-Offs)	0.0	0.0	0.0
Provision for Credit Losses	4.8	3.9	0.7
Balance at End of Period	16.0	11.2	7.3
OTHER FINANCIAL ASSETS
Balance at Beginning of Period	1.0	0.8	0.0
Charge-Offs	0.0	0.0	0.0
Recoveries	0.0	0.0	0.0
Net Recoveries (Charge-Offs)	0.0	0.0	0.0
Provision for Credit Losses	(0.2)	0.2	0.4
Balance at End of Period	0.8	1.0	0.8
TOTAL
Balance at Beginning of Period	184.7	259.9	124.4
Charge-Offs	(6.0)	(0.7)	(9.7)
Recoveries	10.2	7.0	6.5
Net Recoveries (Charge-Offs)	4.2	6.3	(3.2)
Provision for Credit Losses	10.7	(81.5)	125.0
Balance at End of Period	199.6	184.7	259.9
Adjustment
LOANS AND LEASES
Balance at Beginning of Period			(2.2)
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period			8.9
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period			6.6
OTHER FINANCIAL ASSETS
Balance at Beginning of Period			0.4
TOTAL
Balance at Beginning of Period			13.7
Adjusted Balance
LOANS AND LEASES
Balance at Beginning of Period			102.3
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period			28.8
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period			6.6
OTHER FINANCIAL ASSETS
Balance at Beginning of Period			0.4
TOTAL
Balance at Beginning of Period			138.1
Corporate Debt
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	1.4	0.8
Provision for Credit Losses	0.5	0.6
Balance at End of Period	1.9	1.4	$ 0.8
TOTAL
Provision for Credit Losses	$ 1.3	$ 0.0