Allowance for Loan Losses - Impaired Financing Receivables - Average Recorded Investment and Interest Income Recognized (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Financing Receivable, Impaired [Line Items]
Impaired financing receivables, Average Recorded Investment, Total	$ 8,385	$ 11,033
Impaired financing receivables, Interest Income Recognized, Total	125	123
Commercial and Agriculture [Member]
Financing Receivable, Impaired [Line Items]
Impaired financing receivables, Average Recorded Investment, Total	1,697	2,303
Impaired financing receivables, Interest Income Recognized, Total	39	26
Commercial Real Estate Owner Occupied [Member]
Financing Receivable, Impaired [Line Items]
Impaired financing receivables, Average Recorded Investment, Total	1,895	3,328
Impaired financing receivables, Interest Income Recognized, Total	26	47
Commercial Real Estate Non Owner Occupied [Member]
Financing Receivable, Impaired [Line Items]
Impaired financing receivables, Average Recorded Investment, Total	1,862	2,123
Impaired financing receivables, Interest Income Recognized, Total	13	9
Residential Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Impaired financing receivables, Average Recorded Investment, Total	1,753	2,628
Impaired financing receivables, Interest Income Recognized, Total	31	27
Real Estate Construction [Member]
Financing Receivable, Impaired [Line Items]
Impaired financing receivables, Average Recorded Investment, Total		20
Farm Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Impaired financing receivables, Average Recorded Investment, Total	1,175	626
Impaired financing receivables, Interest Income Recognized, Total	16	14
Consumer and Other [Member]
Financing Receivable, Impaired [Line Items]
Impaired financing receivables, Average Recorded Investment, Total	$ 3	$ 5