Allowance for Credit Losses (Changes in Allowance for Credit Losses Related to Loans) (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
Commercial
LOANS
Balance at Beginning of Period	116.2	105.6	142.2
Charge-Offs	5.7	5.3	0.3
Recoveries	0.2	2.7	0.9
Net Recoveries (Charge-Offs)	(5.5)	(2.6)	0.6
Provision for (Release of) Credit Losses	36.1	13.2	(37.2)
Balance at End of Period	146.8	116.2	105.6
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	36.3	31.4	57.6
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	(11.4)	4.9	(26.2)
Balance at End of Period	24.9	36.3	31.4
PERSONAL
LOANS
Balance at Beginning of Period	28.1	32.8	48.5
Charge-Offs	1.8	0.7	0.4
Recoveries	3.5	7.5	6.1
Net Recoveries (Charge-Offs)	1.7	6.8	5.7
Provision for (Release of) Credit Losses	2.1	(11.5)	(21.4)
Balance at End of Period	31.9	28.1	32.8
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	2.2	2.7	3.5
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	(0.2)	(0.5)	(0.8)
Balance at End of Period	$ 2.0	$ 2.2	$ 2.7