Allowance for Loan Losses - Impaired Financing Receivables Excluding PCI Loans - Recorded Investment and Unpaid Principal Balances (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Impaired [Line Items]
Impaired financing receivables, with no related allowance recorded, Recorded Investment	$ 2,311	$ 1,862
Impaired financing receivables, with no related allowance recorded, Unpaid Principal Balance	2,388	2,158
Impaired financing receivables, with an allowance recorded, Recorded Investment	1,417	1,598
Impaired financing receivables, with an allowance recorded, Unpaid Principal Balance	1,520	1,602
Impaired financing receivables, with an allowance recorded, Related Allowance	179	125
Impaired financing receivables, Recorded Investment, Total	3,728	3,460
Impaired financing receivables, Unpaid Principal Balance, Total	3,908	3,760
Impaired financing receivables, Related Allowance, Total	179	125
Commercial and Agriculture [Member]
Financing Receivable, Impaired [Line Items]
Impaired financing receivables, with no related allowance recorded, Recorded Investment	760
Impaired financing receivables, with no related allowance recorded, Unpaid Principal Balance	760
Impaired financing receivables, with an allowance recorded, Recorded Investment	265	438
Impaired financing receivables, with an allowance recorded, Unpaid Principal Balance	365	438
Impaired financing receivables, with an allowance recorded, Related Allowance	60	4
Impaired financing receivables, Recorded Investment, Total	1,025	438
Impaired financing receivables, Unpaid Principal Balance, Total	1,125	438
Impaired financing receivables, Related Allowance, Total	60	4
Commercial Real Estate Owner Occupied [Member]
Financing Receivable, Impaired [Line Items]
Impaired financing receivables, with no related allowance recorded, Recorded Investment	218	693
Impaired financing receivables, with no related allowance recorded, Unpaid Principal Balance	218	913
Impaired financing receivables, with an allowance recorded, Recorded Investment	314	317
Impaired financing receivables, with an allowance recorded, Unpaid Principal Balance	314	317
Impaired financing receivables, with an allowance recorded, Related Allowance	8	6
Impaired financing receivables, Recorded Investment, Total	532	1,010
Impaired financing receivables, Unpaid Principal Balance, Total	532	1,230
Impaired financing receivables, Related Allowance, Total	8	6
Commercial Real Estate Non Owner Occupied [Member]
Financing Receivable, Impaired [Line Items]
Impaired financing receivables, with no related allowance recorded, Recorded Investment	43	44
Impaired financing receivables, with no related allowance recorded, Unpaid Principal Balance	47	48
Impaired financing receivables, Recorded Investment, Total	43	44
Impaired financing receivables, Unpaid Principal Balance, Total	47	48
Residential Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Impaired financing receivables, with no related allowance recorded, Recorded Investment	957	977
Impaired financing receivables, with no related allowance recorded, Unpaid Principal Balance	1,030	1,049
Impaired financing receivables, with an allowance recorded, Recorded Investment	378	383
Impaired financing receivables, with an allowance recorded, Unpaid Principal Balance	381	387
Impaired financing receivables, with an allowance recorded, Related Allowance	105	109
Impaired financing receivables, Recorded Investment, Total	1,335	1,360
Impaired financing receivables, Unpaid Principal Balance, Total	1,411	1,436
Impaired financing receivables, Related Allowance, Total	105	109
Farm Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Impaired financing receivables, with no related allowance recorded, Recorded Investment	333	148
Impaired financing receivables, with no related allowance recorded, Unpaid Principal Balance	333	148
Impaired financing receivables, with an allowance recorded, Recorded Investment	460	460
Impaired financing receivables, with an allowance recorded, Unpaid Principal Balance	460	460
Impaired financing receivables, with an allowance recorded, Related Allowance	6	6
Impaired financing receivables, Recorded Investment, Total	793	608
Impaired financing receivables, Unpaid Principal Balance, Total	793	608
Impaired financing receivables, Related Allowance, Total	$ 6	$ 6