Allowance for Credit Losses (Changes in Allowance for Credit Losses Related to Loans) (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
Balance at End of Period	166.8	138.7	166.8	138.7
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	14.0	5.0	25.1	(3.9)
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
COMMERCIAL
LOANS
Balance at Beginning of Period	124.0	102.5	116.2	105.6
Balance at End of Period	137.4	100.6	137.4	100.6
Charge-Offs	0.5	4.9	4.5	4.9
Recoveries	0.0	0.3	0.1	2.7
Net Recoveries (Charge-Offs)	(0.5)	(4.6)	(4.4)	(2.2)
Provision for (Release of) Credit Losses	13.9	2.7	25.6	(2.8)
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	23.3	40.3	36.3	31.4
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	2.2	(2.8)	(10.8)	6.1
Balance at End of Period	25.5	37.5	25.5	37.5
PERSONAL
LOANS
Balance at Beginning of Period	28.5	35.7	28.1	32.8
Balance at End of Period	29.4	38.1	29.4	38.1
Charge-Offs	0.3	0.5	1.1	0.6
Recoveries	1.1	0.6	2.9	7.0
Net Recoveries (Charge-Offs)	0.8	0.1	1.8	6.4
Provision for (Release of) Credit Losses	0.1	2.3	(0.5)	(1.1)
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	2.7	3.2	2.2	2.7
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	0.1	0.0	0.6	0.5
Balance at End of Period	$ 2.8	$ 3.2	$ 2.8	$ 3.2