Allowance for Credit Losses (Changes in Allowance for Credit Losses Related to Loans) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
LOANS
Balance at Beginning of Period	$ 144.3	$ 138.4
Balance at End of Period	159.9	136.3
Charge-Offs	4.0	0.1
Recoveries	1.1	3.3
Net Recoveries (Charge-Offs)	(2.9)	3.2
Provision for Credit Losses	18.5	(5.3)
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	38.5	34.1
Charge-Offs	0.0	0.0
Recoveries	0.0	0.0
Net Recoveries (Charge-Offs)	0.0	0.0
Provision for Credit Losses	(4.2)	3.4
Balance at End of Period	34.3	37.5
COMMERCIAL
LOANS
Balance at Beginning of Period	116.2	105.6
Balance at End of Period	130.8	102.5
Charge-Offs	4.0	0.0
Recoveries	0.0	2.2
Net Recoveries (Charge-Offs)	(4.0)	2.2
Provision for Credit Losses	18.6	(5.3)
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	36.3	31.4
Charge-Offs	0.0	0.0
Recoveries	0.0	0.0
Net Recoveries (Charge-Offs)	0.0	0.0
Provision for Credit Losses	(5.0)	3.3
Balance at End of Period	31.3	34.7
PERSONAL
LOANS
Balance at Beginning of Period	28.1	32.8
Balance at End of Period	29.1	33.8
Charge-Offs	0.0	0.1
Recoveries	1.1	1.1
Net Recoveries (Charge-Offs)	1.1	1.0
Provision for Credit Losses	(0.1)	0.0
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	2.2	2.7
Charge-Offs	0.0	0.0
Recoveries	0.0	0.0
Net Recoveries (Charge-Offs)	0.0	0.0
Provision for Credit Losses	0.8	0.1
Balance at End of Period	$ 3.0	$ 2.8