Financial Instruments with Off-Balance Sheet Risk (Details) $ in Millions	Mar. 31, 2018 USD ($)
Entity Information [Line Items]
Credit extension commitments	$ 2,216.5
Credit extension commitments beyond one year	795.7
Unused Credit Card Lines
Entity Information [Line Items]
Credit extension commitments	656.7
Standby Letters of Credit
Entity Information [Line Items]
Credit extension commitments	$ 50.2