Allowance for Credit Losses (Changes in Allowance for Credit Losses) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
LOANS
Balance at Beginning of Period	$ 178.7	$ 144.3	$ 144.3
Charge-Offs	(11.1)	(4.0)	(7.5)
Recoveries	0.7	1.1
Net Recoveries (Charge-Offs)	(10.4)	(2.9)
Provision for Credit Losses	(5.9)	18.5
Balance at End of Period	162.4	159.9	178.7
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	26.9	38.5	38.5
Charge-Offs	0.0	0.0
Recoveries	0.0	0.0
Net Recoveries (Charge-Offs)	0.0	0.0
Provision for Credit Losses	(1.7)	(4.2)
Balance at End of Period	25.2	34.3	26.9
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	12.7	16.0	16.0
Charge-Offs	0.0	0.0
Recoveries	0.0	0.0
Net Recoveries (Charge-Offs)	0.0	0.0
Provision for Credit Losses	(0.3)	0.0
Balance at End of Period	12.4	16.0	12.7
OTHER FINANCIAL ASSETS
Balance at Beginning of Period	0.9	0.8	0.8
Charge-Offs	0.0	0.0
Recoveries	0.0	0.0
Net Recoveries (Charge-Offs)	0.0	0.0
Provision for Credit Losses	0.1	0.2
Balance at End of Period	1.0	1.0	0.9
TOTAL
Balance at Beginning of Period	219.2	199.6	199.6
Charge-Offs	(11.1)	(4.0)
Recoveries	0.7	1.1
Net Recoveries (Charge-Offs)	(10.4)	(2.9)
Provision for credit losses	(7.8)	14.5
Balance at End of Period	201.0	211.2	219.2
Corporate Debt
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	0.9	1.9	1.9
Provision for Credit Losses	(0.1)	0.0
Balance at End of Period	0.8	1.9	$ 0.9
TOTAL
Provision for credit losses	$ (0.7)	$ 0.5