Fair Value - Unfunded Loan and Lease Commitments and Letters of Credit (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Unfunded loan and lease commitments and letters of credit
Aggregate commitments to extend credit, standby letters of credit and commercial letters of credit	$ 6,000.0	$ 5,800.0
Level 3
Unfunded loan and lease commitments and letters of credit
Aggregate commitments to extend credit, standby letters of credit and commercial letters of credit	6,000.0	5,800.0
Estimated fair value of unfunded loan and lease commitments and letters of credit	$ 13.7	$ 14.2