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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Financing Receivable, Impaired [Line Items]
Impaired loans, Average Recorded Investment, Total	$ 5,980	$ 8,740	$ 6,166	$ 8,278
Impaired loans, Interest Income Recognized, Total	60	907	123	962
Commercial and Agriculture [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans, Average Recorded Investment, Total	1,643	2,469	1,756	1,937
Impaired loans, Interest Income Recognized, Total	6	8	20	11
Commercial Real Estate Owner Occupied [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans, Average Recorded Investment, Total	1,795	1,630	1,829	1,800
Impaired loans, Interest Income Recognized, Total	26	20	48	45
Commercial Real Estate Non Owner Occupied [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans, Average Recorded Investment, Total	356	1,363	357	1,489
Impaired loans, Interest Income Recognized, Total	2	853	3	857
Residential Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans, Average Recorded Investment, Total	1,572	1,878	1,610	1,799
Impaired loans, Interest Income Recognized, Total	19	20	39	39
Farm Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans, Average Recorded Investment, Total	614	1,398	614	1,250
Impaired loans, Interest Income Recognized, Total	$ 7	6	$ 13	10
Consumer and Other [Member]
Financing Receivable, Impaired [Line Items]
Impaired loans, Average Recorded Investment, Total		$ 2		$ 3