Allowance for Loan Losses - Credit Exposures by Internally Assigned Grades (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	$ 1,167,814	$ 1,154,740
Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	1,139,815	1,125,796
Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	11,351	10,733
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	16,648	18,211
Commercial and Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	179,200	177,101
Commercial and Agriculture [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	174,339	173,783
Commercial and Agriculture [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	2,978	1,509
Commercial and Agriculture [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	1,883	1,809
Commercial Real Estate Owner Occupied [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	217,030	210,121
Commercial Real Estate Owner Occupied [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	208,242	201,228
Commercial Real Estate Owner Occupied [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	3,532	3,512
Commercial Real Estate Owner Occupied [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	5,256	5,381
Commercial Real Estate Non Owner Occupied [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	539,420	523,598
Commercial Real Estate Non Owner Occupied [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	536,178	520,487
Commercial Real Estate Non Owner Occupied [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	2,384	2,023
Commercial Real Estate Non Owner Occupied [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	858	1,088
Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	79,796	78,851
Residential Real Estate [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	73,163	70,908
Residential Real Estate [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	490	580
Residential Real Estate [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	6,143	7,363
Real Estate Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	115,923	124,823
Real Estate Construction [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	115,899	124,769
Real Estate Construction [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	12	13
Real Estate Construction [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	12	41
Farm Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	35,645	38,513
Farm Real Estate [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	31,218	32,908
Farm Real Estate [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	1,955	3,096
Farm Real Estate [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	2,472	2,509
Consumer and Other [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	800	1,733
Consumer and Other [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	776	1,713
Consumer and Other [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	$ 24	$ 20