Allowance for Credit Losses (Changes in Allowance for Credit Losses Related to Loans) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
LOANS
Balance at Beginning of Period	$ 178.7	$ 144.3	$ 144.3
Charge-Offs	(11.1)	(4.0)	(7.5)
Recoveries	0.7	1.1
Net Recoveries (Charge-Offs)	(10.4)	(2.9)
Provision for Credit Losses	(5.9)	18.5
Balance at End of Period	162.4	159.9	178.7
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	26.9	38.5	38.5
Charge-Offs	0.0	0.0
Recoveries	0.0	0.0
Net Recoveries (Charge-Offs)	0.0	0.0
Provision for Credit Losses	(1.7)	(4.2)
Balance at End of Period	25.2	34.3	26.9
COMMERCIAL
LOANS
Balance at Beginning of Period	146.8	116.2	116.2
Charge-Offs	(11.0)	(4.0)	(5.7)
Recoveries	0.0	0.0
Net Recoveries (Charge-Offs)	(11.0)	(4.0)
Provision for Credit Losses	(4.1)	18.6
Balance at End of Period	131.7	130.8	146.8
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	24.9	36.3	36.3
Charge-Offs	0.0	0.0
Recoveries	0.0	0.0
Net Recoveries (Charge-Offs)	0.0	0.0
Provision for Credit Losses	(2.4)	(5.0)
Balance at End of Period	22.5	31.3	24.9
PERSONAL
LOANS
Balance at Beginning of Period	31.9	28.1	28.1
Charge-Offs	(0.1)	0.0	(1.8)
Recoveries	0.7	1.1
Net Recoveries (Charge-Offs)	0.6	1.1
Provision for Credit Losses	(1.8)	(0.1)
Balance at End of Period	30.7	29.1	31.9
UNDRAWN LOAN COMMITMENTS AND STANDBY LETTERS OF CREDIT
Balance at Beginning of Period	2.0	2.2	2.2
Charge-Offs	0.0	0.0
Recoveries	0.0	0.0
Net Recoveries (Charge-Offs)	0.0	0.0
Provision for Credit Losses	0.7	0.8
Balance at End of Period	$ 2.7	$ 3.0	$ 2.0