Loans and Allowance for Loan Losses - Schedule of Aging of the Recorded Investment (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable Recorded Investment Past Due [Line Items]
Nonaccrual	$ 40,276	$ 22,693
Loans Not Past Due	2,057,798	1,355,137
Total Loans	2,103,279	1,383,605
30 to 59 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Financing Receivables, Past Due	3,889	4,097
60 to 89 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Financing Receivables, Past Due	1,316	1,678
Commercial Real Estate [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Nonaccrual	11,607	12,258
Loans Not Past Due	974,748	577,107
Total Loans	987,661	593,108
Commercial Real Estate [Member] | 30 to 59 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Financing Receivables, Past Due	1,284	2,955
Commercial Real Estate [Member] | 60 to 89 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Financing Receivables, Past Due	22	788
Commercial and Industrial [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Nonaccrual	13,217	4,051
Loans Not Past Due	494,045	343,995
Total Loans	507,519	348,465
Commercial and Industrial [Member] | 30 to 59 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Financing Receivables, Past Due	251	419
Commercial and Industrial [Member] | 60 to 89 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Financing Receivables, Past Due	6
Residential Real Estate [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Nonaccrual	6,148	4,285
Loans Not Past Due	367,924	332,887
Total Loans	376,705	338,387
Residential Real Estate [Member] | 30 to 59 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Financing Receivables, Past Due	1,457	368
Residential Real Estate [Member] | 60 to 89 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Financing Receivables, Past Due	1,176	847
Agricultural Real Estate [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Nonaccrual	3,993	1,664
Loans Not Past Due	82,370	36,667
Total Loans	86,486	38,331
Agricultural Real Estate [Member] | 30 to 59 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Financing Receivables, Past Due	123
Consumer [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Nonaccrual	559	432
Loans Not Past Due	48,331	40,124
Total Loans	49,361	40,902
Consumer [Member] | 30 to 59 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Financing Receivables, Past Due	359	303
Consumer [Member] | 60 to 89 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Financing Receivables, Past Due	112	43
Agricultural [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Nonaccrual	4,752	3
Loans Not Past Due	90,380	24,357
Total Loans	95,547	24,412
Agricultural [Member] | 30 to 59 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Financing Receivables, Past Due	$ 415	$ 52