Commitments and Contingencies - Additional Information (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) Lawsuit	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Loss Contingencies [Line Items]
Unused commitments to extend credit	$ 3,172.0	$ 3,131.0
Lease Commitments [Abstract]
Rental expense	$ 9.3	9.8	$ 9.3
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	$ 104.1	111.3
Letters of credit, maturity term - maximum	3 years
Collateral held, fair value	$ 28.4	$ 32.7