Allowance for Credit Losses (Changes in Allowance for Credit Losses) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
LOANS
Balance at Beginning of Period	$ 152.5	$ 138.2	$ 144.3	$ 138.4
Charge-Offs	(0.8)	(5.4)	(5.6)	(5.5)
Recoveries	1.1	0.9	3.0	9.7
Net Recoveries (Charge-Offs)	0.3	(4.5)	(2.6)	4.2
Provision for (Release of) Credit Losses	14.0	5.0	25.1	(3.9)
Balance at End of Period	166.8	138.7	166.8	138.7
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	26.0	43.5	38.5	34.1
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 (Release of) Credit Losses	2.3	(2.8)	(10.2)	6.6
Balance at End of Period	28.3	40.7	28.3	40.7
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	16.7	15.4	16.0	11.2
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 (Release of) Credit Losses	(1.8)	(0.4)	(1.1)	3.8
Balance at End of Period	14.9	15.0	14.9	15.0
OTHER FINANCIAL ASSETS
Balance at Beginning of Period	1.0	1.1	0.8	1.0
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 (Release of) Credit Losses(1)	0.1	(0.4)	0.3	(0.3)
Balance at End of Period	1.1	0.7	1.1	0.7
TOTAL
Balance at Beginning of Period	196.2	198.2	199.6	184.7
Charge-Offs	(0.8)	(5.4)	(5.6)	(5.5)
Recoveries	1.1	0.9	3.0	9.7
Net Recoveries (Charge-Offs)	0.3	(4.5)	(2.6)	4.2
Provision for (release of) credit losses	14.6	1.4	14.1	6.2
Balance at End of Period	211.1	195.1	211.1	195.1
Corporate Debt
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	1.4	1.8	1.9	1.4
Provision for (Release of) Credit Losses	(0.1)	(0.3)	(0.6)	0.1
Balance at End of Period	1.3	1.5	1.3	1.5
TOTAL
Provision for (release of) credit losses	$ (0.6)	$ (0.9)	$ (0.6)	$ 0.8