Allowance for Credit Losses (Changes in Allowance for Credit Losses Related to Loans) (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
COMMERCIAL
LOANS
Balance at Beginning of Period	145.8	136.9	138.5	146.8	146.8
Charge-Offs	(2.0)	0.0	(2.3)	(11.0)	(15.1)
Recoveries	1.3	0.0	1.4	0.0
Net Recoveries (Charge-Offs)	(0.7)	0.0	(0.9)	(11.0)
Provision for Credit Losses	(11.2)	15.7	(3.7)	16.8
Balance at End of Period	133.9	152.6	133.9	152.6	138.5
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	32.5	27.2	28.3	24.9	24.9
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)	(2.7)	1.6	(0.4)
Balance at End of Period	29.9	24.5	29.9	24.5	28.3
PERSONAL
LOANS
Balance at Beginning of Period	34.7	30.8	29.5	31.9	31.9
Charge-Offs	(0.1)	0.0	(0.2)	(0.4)	(0.4)
Recoveries	0.4	2.4	1.2	3.5
Net Recoveries (Charge-Offs)	0.3	2.4	1.0	3.1
Provision for Credit Losses	(4.5)	(1.0)	0.0	(2.8)
Balance at End of Period	30.5	32.2	30.5	32.2	29.5
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	2.2	2.3	2.1	2.0	2.0
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	0.0	(0.3)	0.1	0.0
Balance at End of Period	$ 2.2	$ 2.0	$ 2.2	$ 2.0	$ 2.1