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, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023
LOANS
Balance at Beginning of Period	$ 167.7	$ 152.5	$ 178.7	$ 144.3	$ 144.3
Charge-Offs	0.0	(0.8)	(11.4)	(5.6)	(7.5)
Recoveries	2.4	1.1	3.5	3.0
Net Recoveries (Charge-Offs)	2.4	0.3	(7.9)	(2.6)
Provision for Credit Losses	14.7	14.0	14.0	25.1
Balance at End of Period	184.8	166.8	184.8	166.8	178.7
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	29.5	26.0	26.9	38.5	38.5
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	(3.0)	2.3	(0.4)	(10.2)
Balance at End of Period	26.5	28.3	26.5	28.3	26.9
COMMERCIAL
LOANS
Balance at Beginning of Period	136.9	124.0	146.8	116.2	116.2
Charge-Offs	0.0	(0.5)	(11.0)	(4.5)	(5.7)
Recoveries	0.0	0.0	0.0	0.1
Net Recoveries (Charge-Offs)	0.0	(0.5)	(11.0)	(4.4)
Provision for Credit Losses	15.7	13.9	16.8	25.6
Balance at End of Period	152.6	137.4	152.6	137.4	146.8
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	27.2	23.3	24.9	36.3	36.3
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	(2.7)	2.2	(0.4)	(10.8)
Balance at End of Period	24.5	25.5	24.5	25.5	24.9
PERSONAL
LOANS
Balance at Beginning of Period	30.8	28.5	31.9	28.1	28.1
Charge-Offs	0.0	(0.3)	(0.4)	(1.1)	(1.8)
Recoveries	2.4	1.1	3.5	2.9
Net Recoveries (Charge-Offs)	2.4	0.8	3.1	1.8
Provision for Credit Losses	(1.0)	0.1	(2.8)	(0.5)
Balance at End of Period	32.2	29.4	32.2	29.4	31.9
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	2.3	2.7	2.0	2.2	2.2
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	(0.3)	0.1	0.0	0.6
Balance at End of Period	$ 2.0	$ 2.8	$ 2.0	$ 2.8	$ 2.0