Allowance for Credit Losses (Changes in Allowance for Credit Losses) (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
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	10.9	16.7	12.7	16.0	16.0
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	(3.4)	(1.8)	(5.2)	(1.1)
Balance at End of Period	7.5	14.9	7.5	14.9	12.7
OTHER FINANCIAL ASSETS
Balance at Beginning of Period	0.9	1.0	0.9	0.8	0.8
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	0.1	0.1	0.1	0.3
Balance at End of Period	1.0	1.1	1.0	1.1	0.9
TOTAL
Balance at Beginning of Period	209.0	196.2	219.2	199.6	199.6
Charge-Offs	0.0	(0.8)	(11.4)	(5.6)
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	8.4	14.6	8.5	14.1
Balance at End of Period	219.8	211.1	219.8	211.1	219.2
Corporate Debt
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	0.5	1.4	0.9	1.9	1.9
Provision for Credit Losses	0.0	(0.1)	(0.4)	(0.6)
Balance at End of Period	0.5	1.3	0.5	1.3	$ 0.9
TOTAL
Provision for credit losses	$ (0.4)	$ (0.6)	$ (1.0)	$ (0.6)