Allowance for Credit Losses (Changes in Allowance for Credit Losses) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024
LOANS
Balance at Beginning of Period	$ 180.5	$ 167.7	$ 168.0	$ 178.7	$ 178.7
Charge-Offs	(2.1)	0.0	(2.5)	(11.4)	(15.5)
Recoveries	1.7	2.4	2.6	3.5
Net Recoveries (Charge-Offs)	(0.4)	2.4	0.1	(7.9)
Provision for Credit Losses	(15.7)	14.7	(3.7)	14.0
Balance at End of Period	164.4	184.8	164.4	184.8	168.0
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	34.7	29.5	30.4	26.9	26.9
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	(2.6)	(3.0)	1.7	(0.4)
Balance at End of Period	32.1	26.5	32.1	26.5	30.4
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	6.9	10.9	6.5	12.7	12.7
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	1.8	(3.4)	2.2	(5.2)
Balance at End of Period	8.7	7.5	8.7	7.5	6.5
OTHER FINANCIAL ASSETS
Balance at Beginning of Period	1.1	0.9	1.0	0.9	0.9
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	0.4	0.1	0.5	0.1
Balance at End of Period	1.5	1.0	1.5	1.0	1.0
TOTAL
Balance at Beginning of Period	223.2	209.0	205.9	219.2	219.2
Charge-Offs	(2.1)	0.0	(2.5)	(11.4)
Recoveries	1.7	2.4	2.6	3.5
Net Recoveries (Charge-Offs)	(0.4)	2.4	0.1	(7.9)
Provision for credit losses	(16.1)	8.4	0.7	8.5
Balance at End of Period	206.7	219.8	206.7	219.8	205.9
TOTAL	$ 206.7	$ 219.8	$ 206.7	$ 219.8	$ 205.9