Allowance for Credit Losses (Changes in Allowance for Credit Losses Related to Loans) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
LOANS
Balance at Beginning of Period	$ 168.0	$ 178.7	$ 144.3
Charge-Offs	3.9	15.5	7.5
Recoveries	3.6	4.2	3.7
Net Recoveries (Charge-Offs)	(0.3)	(11.3)	(3.8)
Provision for Credit Losses	(3.4)	0.6	38.2
Balance at End of Period	164.3	168.0	178.7
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	30.4	26.9	38.5
Charge-Offs	0.0	0.0	0.0
Recoveries	0.0	0.0	0.0
Net Recoveries (Charge-Offs)	0.0	0.0	0.0
Provision for Credit Losses	(7.1)	3.5	(11.6)
Balance at End of Period	23.3	30.4	26.9
Commercial
LOANS
Balance at Beginning of Period	138.5	146.8	116.2
Charge-Offs	3.5	15.1	5.7
Recoveries	2.2	0.0	0.2
Net Recoveries (Charge-Offs)	(1.3)	(15.1)	(5.5)
Provision for Credit Losses	(4.7)	6.8	36.1
Balance at End of Period	132.5	138.5	146.8
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	28.3	24.9	36.3
Charge-Offs	0.0	0.0	0.0
Recoveries	0.0	0.0	0.0
Net Recoveries (Charge-Offs)	0.0	0.0	0.0
Provision for Credit Losses	(6.6)	3.4	(11.4)
Balance at End of Period	21.7	28.3	24.9
Personal
LOANS
Balance at Beginning of Period	29.5	31.9	28.1
Charge-Offs	0.4	0.4	1.8
Recoveries	1.4	4.2	3.5
Net Recoveries (Charge-Offs)	1.0	3.8	1.7
Provision for Credit Losses	1.3	(6.2)	2.1
Balance at End of Period	31.8	29.5	31.9
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	2.1	2.0	2.2
Charge-Offs	0.0	0.0	0.0
Recoveries	0.0	0.0	0.0
Net Recoveries (Charge-Offs)	0.0	0.0	0.0
Provision for Credit Losses	(0.5)	0.1	(0.2)
Balance at End of Period	$ 1.6	$ 2.1	$ 2.0