Financial Instruments with Off-Balance Sheet Risk (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Entity Information [Line Items]
Off-Balance Sheet, Credit Loss, Liability	$ 3.4	$ 2.3	$ 3.4	$ 2.3	$ 3.4	$ 3.7	$ 2.1	$ 0.0
Provision for (reversal of) credit loss expense	0.0	(19.3)		(48.5)
Credit Extension Commitments	2,522.4		2,522.4			2,280.0
Unused Credit Card Lines
Entity Information [Line Items]
Credit Extension Commitments	681.8		681.8			682.8
Standby Letter of Credit
Entity Information [Line Items]
Credit Extension Commitments	59.3		59.3			59.0
Cumulative effect, period of adoption, adjusted balance
Entity Information [Line Items]
Off-Balance Sheet, Credit Loss, Liability		2.3		2.3		$ 0.0		$ 0.0
Provision for (reversal of) credit loss expense	$ 0.0	$ 0.2	$ (0.3)	$ 0.0