Allowance for Credit Losses (Changes in Allowance for Credit Losses Related to Loans) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2026	Mar. 31, 2025	Dec. 31, 2025
LOANS
Balance at Beginning of Period	$ 164.3	$ 168.0	$ 168.0
Charge-Offs	(0.4)	(0.3)	(3.9)
Recoveries	0.3	0.5
Net Recoveries (Charge-Offs)	(0.1)	0.2
Provision for Credit Losses	(3.1)	(1.1)
Balance at End of Period	161.1	167.1	164.3
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	23.3	30.4	30.4
Charge-Offs	0.0	0.0
Recoveries	0.0	0.0
Net Recoveries (Charge-Offs)	0.0	0.0
Provision for Credit Losses	2.2	2.4
Balance at End of Period	25.5	32.8	23.3
COMMERCIAL
LOANS
Balance at Beginning of Period	132.5	138.5	138.5
Charge-Offs	(0.4)	(0.3)	(3.5)
Recoveries	0.0	0.1
Net Recoveries (Charge-Offs)	(0.4)	(0.2)
Provision for Credit Losses	(0.6)	(4.7)
Balance at End of Period	131.5	133.6	132.5
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	21.7	28.3	28.3
Charge-Offs	0.0	0.0
Recoveries	0.0	0.0
Net Recoveries (Charge-Offs)	0.0	0.0
Provision for Credit Losses	(0.2)	2.4
Balance at End of Period	21.5	30.7	21.7
PERSONAL
LOANS
Balance at Beginning of Period	31.8	29.5	29.5
Charge-Offs	0.0	0.0	(0.4)
Recoveries	0.3	0.4
Net Recoveries (Charge-Offs)	0.3	0.4
Provision for Credit Losses	(2.5)	3.6
Balance at End of Period	29.6	33.5	31.8
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	1.6	2.1	2.1
Charge-Offs	0.0	0.0
Recoveries	0.0	0.0
Net Recoveries (Charge-Offs)	0.0	0.0
Provision for Credit Losses	2.4	0.0
Balance at End of Period	$ 4.0	$ 2.1	$ 1.6