Fair Value - Unfunded Loan and Lease Commitments and Letters of Credit (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Unfunded loan and lease commitments and letters of credit
Aggregate commitments to extend credit, standby letters of credit and commercial letters of credit	$ 6,100.0	$ 6,100.0
Level 3
Unfunded loan and lease commitments and letters of credit
Estimated fair value of unfunded loan and lease commitments and letters of credit	$ 42.3	$ 14.4