Financial Instruments with Off-Balance Sheet Risk (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Entity Information
Credit extension commitments	$ 2,179.5	$ 1,600.0
Unused credit card lines	775.0	424.5
Unused Credit Card Lines
Entity Information
Credit extension commitments	635.3	544.9
Standby Letters of Credit
Entity Information
Credit extension commitments	$ 50.5	$ 50.6