Loans and Allowance for Loan Losses - Nonaccrual Loans, Excluding Loans Acquired, Segregated by Class of Loans (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans in nonaccrual status	$ 40,505	$ 45,642
Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans in nonaccrual status	3,645	4,775
Consumer | Credit cards
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans in nonaccrual status	328	170
Consumer | Other consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans in nonaccrual status	3,317	4,605
Real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans in nonaccrual status	22,436	31,727
Real estate | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans in nonaccrual status	1,289	2,242
Real estate | Single family residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans in nonaccrual status	11,508	13,431
Real estate | Other commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans in nonaccrual status	9,639	16,054
Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans in nonaccrual status	14,424	9,140
Commercial | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans in nonaccrual status	11,932	6,980
Commercial | Agricultural
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans in nonaccrual status	$ 2,492	$ 2,160