Allowance for Credit Losses (Changes in Allowance for Credit Losses) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023
LOANS
Balance at Beginning of Period	$ 162.4	$ 159.9	$ 178.7	$ 144.3	$ 144.3
Charge-Offs	(0.3)	(0.8)	(11.4)	(4.8)	(7.5)
Recoveries	0.4	0.8	1.1	1.9
Net Recoveries (Charge-Offs)	0.1	0.0	(10.3)	(2.9)
Provision for Credit Losses	5.2	(7.4)	(0.7)	11.1
Balance at End of Period	167.7	152.5	167.7	152.5	178.7
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	25.2	34.3	26.9	38.5	38.5
Charge-Offs	0.0	0.0	0.0	0.0
Recoveries	0.0	0.0	0.0	0.0
Net Recoveries (Charge-Offs)	0.0	0.0	0.0	0.0
Provision for Credit Losses	4.3	(8.3)	2.6	(12.5)
Balance at End of Period	29.5	26.0	29.5	26.0	26.9
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	12.4	16.0	12.7	16.0	16.0
Charge-Offs	0.0	0.0	0.0	0.0
Recoveries	0.0	0.0	0.0	0.0
Net Recoveries (Charge-Offs)	0.0	0.0	0.0	0.0
Provision for Credit Losses	(1.5)	0.7	(1.8)	0.7
Balance at End of Period	10.9	16.7	10.9	16.7	12.7
OTHER FINANCIAL ASSETS
Balance at Beginning of Period	1.0	1.0	0.9	0.8	0.8
Charge-Offs	0.0	0.0	0.0	0.0
Recoveries	0.0	0.0	0.0	0.0
Net Recoveries (Charge-Offs)	0.0	0.0	0.0	0.0
Provision for Credit Losses	(0.1)	0.0	0.0	0.2
Balance at End of Period	0.9	1.0	0.9	1.0	0.9
TOTAL
Balance at Beginning of Period	201.0	211.2	219.2	199.6	199.6
Charge-Offs	(0.3)	(0.8)	(11.4)	(4.8)
Recoveries	0.4	0.8	1.1	1.9
Net Recoveries (Charge-Offs)	0.1	0.0	(10.3)	(2.9)
Provision for credit losses	7.9	(15.0)	0.1	(0.5)
Balance at End of Period	209.0	196.2	209.0	196.2	219.2
Corporate Debt
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	0.8	1.9	0.9	1.9	1.9
Provision for Credit Losses	(0.3)	(0.5)	(0.4)	(0.5)
Balance at End of Period	0.5	1.4	0.5	1.4	$ 0.9
TOTAL
Provision for credit losses	$ 0.1	$ (0.5)	$ (0.6)	$ 0.0