Contingencies (Details) $ in Millions	6 Months Ended
	Jun. 30, 2015 USD ($) Lawsuit	Jun. 30, 2014 USD ($)
Loss Contingencies [Line Items]
Unused commitments to extend credit	$ 2,540.0	$ 2,223.0
Standby Letters of Credit [Member]
Loss Contingencies [Line Items]
Maximum potential exposure to credit loss in the event of nonperformance	$ 134.0	$ 147.5
Letters of credit, maturity term - maximum	3 years
Collateral held, fair value	$ 28.7
Pending or Threatened Litigation, Stanford Financial Group [Member]
Legal Proceedings [Abstract]
Lawsuits naming entity as defendant, number | Lawsuit	2