Contingencies (Details) $ in Millions	9 Months Ended
	Sep. 30, 2015 USD ($) Lawsuit	Sep. 30, 2014 USD ($)
Loss Contingencies [Line Items]
Unused commitments to extend credit	$ 2,724.0	$ 2,443.0
Standby Letters of Credit [Member]
Loss Contingencies [Line Items]
Maximum potential exposure to credit loss in the event of nonperformance	$ 132.2	137.1
Letters of credit, maturity term - maximum	3 years
Collateral held, fair value	$ 31.7	$ 32.7
Pending or Threatened Litigation, Stanford Financial Group [Member]
Legal Proceedings [Abstract]
Lawsuits naming entity as defendant, number | Lawsuit	2