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, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023
LOANS
Balance at Beginning of Period	$ 162.4	$ 159.9	$ 178.7	$ 144.3	$ 144.3
Charge-Offs	(0.3)	(0.8)	(11.4)	(4.8)	(7.5)
Recoveries	0.4	0.8	1.1	1.9
Net Recoveries (Charge-Offs)	0.1	0.0	(10.3)	(2.9)
Provision for Credit Losses	5.2	(7.4)	(0.7)	11.1
Balance at End of Period	167.7	152.5	167.7	152.5	178.7
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	25.2	34.3	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	4.3	(8.3)	2.6	(12.5)
Balance at End of Period	29.5	26.0	29.5	26.0	26.9
COMMERCIAL
LOANS
Balance at Beginning of Period	131.7	130.8	146.8	116.2	116.2
Charge-Offs	0.0	0.0	(11.0)	(4.0)	(5.7)
Recoveries	0.0	0.1	0.0	0.1
Net Recoveries (Charge-Offs)	0.0	0.1	(11.0)	(3.9)
Provision for Credit Losses	5.2	(6.9)	1.1	11.7
Balance at End of Period	136.9	124.0	136.9	124.0	146.8
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	22.5	31.3	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	4.7	(8.0)	2.3	(13.0)
Balance at End of Period	27.2	23.3	27.2	23.3	24.9
PERSONAL
LOANS
Balance at Beginning of Period	30.7	29.1	31.9	28.1	28.1
Charge-Offs	(0.3)	(0.8)	(0.4)	(0.8)	(1.8)
Recoveries	0.4	0.7	1.1	1.8
Net Recoveries (Charge-Offs)	0.1	(0.1)	0.7	1.0
Provision for Credit Losses	0.0	(0.5)	(1.8)	(0.6)
Balance at End of Period	30.8	28.5	30.8	28.5	31.9
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	2.7	3.0	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	(0.4)	(0.3)	0.3	0.5
Balance at End of Period	$ 2.3	$ 2.7	$ 2.3	$ 2.7	$ 2.0