Financial Instruments with Off-Balance Sheet Risk (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Entity Information [Line Items]
Off-Balance Sheet, Credit Loss, Liability	$ 3.4	$ 2.1	$ 3.7	$ 0.0
Provision for (reversal of) credit loss expense	4.8	(29.2)
Credit Extension Commitments	2,302.1		2,280.0
Unused Credit Card Lines
Entity Information [Line Items]
Credit Extension Commitments	684.2		682.8
Standby Letter of Credit
Entity Information [Line Items]
Credit Extension Commitments	61.9		59.0
Cumulative effect, period of adoption, adjusted balance
Entity Information [Line Items]
Off-Balance Sheet, Credit Loss, Liability			$ 0.0	$ 2.3
Provision for (reversal of) credit loss expense	$ (0.3)	$ (0.2)