Financial Instruments with Off-Balance Sheet Risk (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Entity Information
Unused credit card lines		$ 2,067.0
Off-Balance Sheet, Credit Loss, Liability	$ 3.7	0.0
Off-Balance Sheet, Credit Loss, Liability, Credit Loss Expense (Reversal)	1.4	0.0
Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Entity Information
Off-Balance Sheet, Credit Loss, Liability	2.3	0.0
Unused Credit Card Lines
Entity Information
Credit extension commitments	682.8	671.8
Unused credit card lines	2,280.0
Standby Letters of Credit
Entity Information
Credit extension commitments	$ 59.0	$ 42.7