Commitments and Contingencies - Additional Information (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) Lawsuit	Dec. 31, 2016 USD ($)
Loss Contingencies [Line Items]
Unused commitments to extend credit	$ 3,918.0	$ 3,172.0
Lease Commitments [Abstract]
Rental expense	9.2	$ 9.3	$ 9.8
Pending or Threatened Litigation, Stanford Financial Group [Member]
Legal Proceedings [Abstract]
Lawsuits naming entity as defendant, number | Lawsuit		3
Standby Letters of Credit [Member]
Loss Contingencies [Line Items]
Potential exposure to credit loss in the event of nonperformance	$ 100.2	$ 104.1
Letters of credit, maturity term - maximum	3 years
Collateral held, fair value	$ 29.8	$ 28.4