Loans and Allowance for Loan Losses - Schedule of Allowance for Loan Losses by Portfolio Segment (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	$ 7,042		$ 5,071	$ 4,645
Loan collectively evaluated for impairment	38,256		36,806	35,775
Total allowance for loan losses	45,298	$ 45,060	41,877	40,420	$ 38,999	$ 36,824
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	3,983		2,030	1,666
Loan collectively evaluated for impairment	9,733		10,614	10,195
Total allowance for loan losses	13,716	14,026	12,644	11,861	11,456	7,990
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	41		70	47
Loan collectively evaluated for impairment	1,027		1,121	867
Total allowance for loan losses	1,068	1,451	1,191	914	392	527
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	2,566		2,827	2,844
Loan collectively evaluated for impairment	23,655		21,548	20,954
Total allowance for loan losses	26,221	25,644	24,375	23,798	24,342	26,657
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	452		144	88
Loan collectively evaluated for impairment	3,841		3,523	3,759
Total allowance for loan losses	$ 4,293	$ 3,939	$ 3,667	$ 3,847	$ 2,809	$ 1,650