Loans - Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) Score	Dec. 31, 2016 USD ($) Score	Dec. 31, 2015 USD ($)
Financing Receivable, Recorded Investment, Past Due [Line Items]
Less unamortized debt issuance costs	$ 35,900	$ 35,500
Interest income that would be recognized on impaired loans if on full accrual basis	2,200	1,600	$ 1,300
Loans greater than 90 days delinquent and still accruing	6,313	6,189
Troubled Debt Restructured during the last twelve months and subsequently redefaulting during the twelve months ended	2,900	232
Troubled debt restructured loans	20,700	15,400
Change-offs of troubled debt restructured loans	$ 238	$ (931)
Weighted average FICO score for the indirect loan portfolio (as a score) | Score	762	752
Automobile Loan
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans pledged to FRB	$ 330,000	$ 311,400
Residential mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans greater than 90 days delinquent and still accruing	268	1,577
Covered
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans and leases receivable, net reported amount, covered	2,300	15,900
Loans Insured or Guaranteed by US Government Authorities
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 19,500	$ 7,800