Loans and Allowance for Loan Losses - Schedule of Allowance for Loan Losses by Portfolio Segment (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2013
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	$ 4,645		$ 4,213	$ 4,634
Loans collectively evaluated for impairment	35,775		32,611	31,754
Total allowance for loan losses	40,420	$ 38,999	36,824	36,388	$ 35,892	$ 33,900
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	1,666		1,171	1,160
Loans collectively evaluated for impairment	10,195		6,819	6,326
Total allowance for loan losses	11,861	11,456	7,990	7,486	6,861	6,440
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	47		57	37
Loans collectively evaluated for impairment	867		470	348
Total allowance for loan losses	914	392	527	385	364	383
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	2,844		2,867	3,256
Loans collectively evaluated for impairment	20,954		23,790	23,043
Total allowance for loan losses	23,798	24,342	26,657	26,299	26,561	24,940
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	88		118	181
Loans collectively evaluated for impairment	3,759		1,532	2,037
Total allowance for loan losses	$ 3,847	$ 2,809	$ 1,650	$ 2,218	$ 2,106	$ 2,137