Allowance for Loan Losses - Impaired Loans - Average Recorded Investment and Interest Income Recognized (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Financing Receivable, Impaired [Line Items]
Impaired loans, Average Recorded Investment, Total	$ 8,740	$ 10,395	$ 8,278	$ 10,646
Impaired loans, Interest Income Recognized, Total	122	124	247	247
Commercial and Agriculture [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans, Average Recorded Investment, Total	2,469	1,762	1,937	1,943
Impaired loans, Interest Income Recognized, Total	41	11	80	37
Commercial Real Estate Owner Occupied [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans, Average Recorded Investment, Total	1,630	3,277	1,800	3,300
Impaired loans, Interest Income Recognized, Total	24	50	50	97
Commercial Real Estate Non Owner Occupied [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans, Average Recorded Investment, Total	1,363	1,992	1,489	2,053
Impaired loans, Interest Income Recognized, Total	13	10	26	19
Residential Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans, Average Recorded Investment, Total	1,878	2,471	1,799	2,752
Impaired loans, Interest Income Recognized, Total	29	39	60	66
Real Estate Construction [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans, Average Recorded Investment, Total		40		27
Farm Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans, Average Recorded Investment, Total	1,398	848	1,250	566
Impaired loans, Interest Income Recognized, Total	15	14	31	28
Consumer and Other [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans, Average Recorded Investment, Total	$ 2	$ 5	$ 3	$ 5