Commitments and Contingencies - Additional Information (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) Lawsuit	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Loss Contingencies [Line Items]
Unused commitments to extend credit	$ 3,131.0	$ 2,873.0
Lease Commitments [Abstract]
Rental expense	$ 9.8	9.3	$ 8.5
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]
Letters of credit, maturity term - maximum	3 years
Collateral held, fair value	$ 32.7	30.5
Standby Letters of Credit [Member] | Maximum [Member]
Loss Contingencies [Line Items]
Potential exposure to credit loss in the event of nonperformance	$ 111.3	$ 113.6