Commitments and Contingencies - Additional Information (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) Lawsuit	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Loss Contingencies [Line Items]
Unused commitments to extend credit	$ 2,873.0	$ 2,489.0
Lease Commitments [Abstract]
Rental expense	9.3	8.5	$ 8.4
Standby Letters of Credit [Member]
Loss Contingencies [Line Items]
Maximum potential exposure to credit loss in the event of nonperformance	$ 113.6	132.8
Letters of credit, maturity term - maximum	3 years
Collateral held, fair value	$ 30.5	$ 31.5
Pending or Threatened Litigation, Stanford Financial Group [Member]
Legal Proceedings [Abstract]
Lawsuits naming entity as defendant, number | Lawsuit	2