Allowance for Credit Losses (Changes in Allowance for Credit Losses) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
LOANS
Balance at Beginning of Period	$ 144.3	$ 138.4
Charge-Offs	(4.0)	(0.1)
Recoveries	1.1	3.3
Net Recoveries (Charge-Offs)	(2.9)	3.2
Provision for Credit Losses	18.5	(5.3)
Balance at End of Period	159.9	136.3
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	38.5	34.1
Charge-Offs	0.0	0.0
Recoveries	0.0	0.0
Net Recoveries (Charge-Offs)	0.0	0.0
Provision for Credit Losses	(4.2)	3.4
Balance at End of Period	34.3	37.5
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	16.0	11.2
Charge-Offs	0.0	0.0
Recoveries	0.0	0.0
Net Recoveries (Charge-Offs)	0.0	0.0
Provision for Credit Losses	0.0	3.4
Balance at End of Period	16.0	14.6
OTHER FINANCIAL ASSETS
Balance at Beginning of Period	0.8	1.0
Charge-Offs	0.0	0.0
Recoveries	0.0	0.0
Net Recoveries (Charge-Offs)	0.0	0.0
Provision for Credit Losses	0.2	0.1
Balance at End of Period	1.0	1.1
TOTAL
Balance at Beginning of Period	199.6	184.7
Charge-Offs	(4.0)	(0.1)
Recoveries	1.1	3.3
Net Recoveries (Charge-Offs)	(2.9)	3.2
Provision for credit losses	14.5	1.6
Balance at End of Period	211.2	189.5
Corporate Debt
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	1.9	1.4
Provision for Credit Losses	0.0	0.2
Balance at End of Period	1.9	1.6
TOTAL
Provision for credit losses	$ 0.5	$ 0.4