Loans and Allowance for Loan Losses - Schedule of Allowance for Loan Losses by Portfolio Segment (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	$ 3,860	$ 4,213
Loans collectively evaluated for impairment	33,968	32,611
Total allowance for loan losses	37,828	36,824	34,693	33,900
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	1,079	1,171
Loans collectively evaluated for impairment	9,296	6,819
Total allowance for loan losses	10,375	7,990	6,645	6,440
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	55	57
Loans collectively evaluated for impairment	418	470
Total allowance for loan losses	473	527	285	383
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	2,617	2,867
Loans collectively evaluated for impairment	22,622	23,790
Total allowance for loan losses	25,239	26,657	25,629	24,940
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	109	118
Loans collectively evaluated for impairment	1,632	1,532
Total allowance for loan losses	$ 1,741	$ 1,650	$ 2,134	$ 2,137