Loans and Allowance for Loan Losses - Schedule of Risk Category of Loans by Class of Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 2,600,924	$ 2,575,408
Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	2,505,051	2,489,353
Classified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	95,873	86,055
Commercial Real Estate [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	1,183,305	1,231,217
Commercial Real Estate [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	1,165,707	1,194,240
Commercial Real Estate [Member] | Classified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	17,598	36,977
Commercial and Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	585,797	601,782
Commercial and Industrial [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	535,939	572,300
Commercial and Industrial [Member] | Classified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	49,858	29,482
Residential Real Estate [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	531,257	446,060
Residential Real Estate [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	521,906	440,704
Residential Real Estate [Member] | Classified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	9,351	5,356
Agricultural Real Estate [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	143,718	139,332
Agricultural Real Estate [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	130,920	129,285
Agricultural Real Estate [Member] | Classified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	12,798	10,047
Consumer [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	70,944	62,894
Consumer [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	70,320	61,976
Consumer [Member] | Classified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	624	918
Agricultural [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	85,903	94,123
Agricultural [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	80,259	90,848
Agricultural [Member] | Classified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 5,644	$ 3,275