Contingencies (Details) (USD $) In Millions, unless otherwise specified			6 Months Ended			3 Months Ended
	Jun. 30, 2014 Member Customer	Jun. 30, 2013	Jun. 30, 2014 Standby Letters of Credit [Member]	Jun. 30, 2013 Standby Letters of Credit [Member]	Jun. 30, 2014 Pending or Threatened Litigation, Stanford Financial Group [Member] Lawsuit	Jun. 30, 2013 Pending or Threatened Litigation, Overdraft Fees [Member]	Jun. 30, 2014 Pending or Threatened Litigation, Overdraft Fees [Member] Lawsuit
Loss Contingencies [Line Items]
Unused commitments to extend credit	$ 2,223	$ 1,963
Maximum potential exposure to credit loss in the event of nonperformance			147.5	164.0
Letters of credit, maturity term - maximum			3 years
Collateral held, fair value			38.0
Legal Proceedings [Abstract]
Lawsuits naming entity as defendant, number					2		2
Litigation settlement						4.0
Litigation settlement net of taxes						$ 2.5
Potential claims resolved by settlement	100,000
Number of customers that excluded themselves from the class action settlement	16