Allowance for Credit Losses (Changes in Allowance for Credit Losses Related to Loans) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025	Dec. 31, 2025
LOANS
Balance at Beginning of Period	$ 161.1	$ 167.1	$ 164.3	$ 168.0	$ 168.0
Charge-Offs	(0.2)	(0.1)	(0.6)	(0.4)	(3.9)
Recoveries	0.6	0.4	0.9	0.9
Net Recoveries (Charge-Offs)	0.4	0.3	0.3	0.5
Provision for Credit Losses	(0.6)	13.1	(3.7)	12.0
Balance at End of Period	160.9	180.5	160.9	180.5	164.3
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	25.5	32.8	23.3	30.4	30.4
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	(3.6)	1.9	(1.4)	4.3
Balance at End of Period	21.9	34.7	21.9	34.7	23.3
COMMERCIAL
LOANS
Balance at Beginning of Period	131.5	133.6	132.5	138.5	138.5
Charge-Offs	0.0	0.0	(0.4)	(0.3)	(3.5)
Recoveries	0.0	0.0	0.0	0.1
Net Recoveries (Charge-Offs)	0.0	0.0	(0.4)	(0.2)
Provision for Credit Losses	(0.8)	12.2	(1.4)	7.5
Balance at End of Period	130.7	145.8	130.7	145.8	132.5
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	21.5	30.7	21.7	28.3	28.3
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	(3.6)	1.8	(3.8)	4.2
Balance at End of Period	17.9	32.5	17.9	32.5	21.7
PERSONAL
LOANS
Balance at Beginning of Period	29.6	33.5	31.8	29.5	29.5
Charge-Offs	(0.2)	(0.1)	(0.2)	(0.1)	(0.4)
Recoveries	0.6	0.4	0.9	0.8
Net Recoveries (Charge-Offs)	0.4	0.3	0.7	0.7
Provision for Credit Losses	0.2	0.9	(2.3)	4.5
Balance at End of Period	30.2	34.7	30.2	34.7	31.8
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	4.0	2.1	1.6	2.1	2.1
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	0.1	2.4	0.1
Balance at End of Period	$ 4.0	$ 2.2	$ 4.0	$ 2.2	$ 1.6