Allowance for Credit Losses (Changes in Allowance for Credit Losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
LOANS
Balance at Beginning of Period	$ 144.3	$ 138.4	$ 190.7
Charge-Offs	(7.5)	(6.0)	(0.7)
Recoveries	3.7	10.2	7.0
Net Recoveries (Charge-Offs)	(3.8)	4.2	6.3
Provision for (Release of) Credit Losses	38.2	1.7	(58.6)
Balance at End of Period	178.7	144.3	138.4
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	38.5	34.1	61.1
Charge-Offs	0.0	0.0	0.0
Recoveries	0.0	0.0	0.0
Net Recoveries (Charge-Offs)	0.0	0.0	0.0
Provision for (Release of) Credit Losses	(11.6)	4.4	(27.0)
Balance at End of Period	26.9	38.5	34.1
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	16.0	11.2	7.3
Charge-Offs	(1.2)	0.0	0.0
Recoveries	0.0	0.0	0.0
Net Recoveries (Charge-Offs)	(1.2)	0.0	0.0
Provision for (Release of) Credit Losses	(2.1)	4.8	3.9
Balance at End of Period	12.7	16.0	11.2
OTHER FINANCIAL ASSETS
Balance at Beginning of Period	0.8	1.0	0.8
Charge-Offs	0.0	0.0	0.0
Recoveries	0.0	0.0	0.0
Net Recoveries (Charge-Offs)	0.0	0.0	0.0
Provision for (Release of) Credit Losses	0.1	(0.2)	0.2
Balance at End of Period	0.9	0.8	1.0
TOTAL
Balance at Beginning of Period	199.6	184.7	259.9
Charge-Offs	(8.7)	(6.0)	(0.7)
Recoveries	3.7	10.2	7.0
Net Recoveries (Charge-Offs)	(5.0)	4.2	6.3
Provision for (Release of) Credit Losses	24.6	10.7	(81.5)
Balance at End of Period	219.2	199.6	184.7
Corporate Debt
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	1.9	1.4	0.8
Charge-Offs	0.0
Recoveries	0.0
Provision for (Release of) Credit Losses	(1.0)	0.5	0.6
Balance at End of Period	0.9	1.9	$ 1.4
TOTAL
Provision for (Release of) Credit Losses	$ (0.1)	$ 1.3