Allowance for Loan Losses - Summary of Allowance for Loan Losses (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Allowance for Loan Losses:
Allowance for loan losses, beginning of year	$ 29,585,000	$ 28,543,000	$ 26,255,000
Charge-offs	(238,000)	(454,000)	(833,000)
Recoveries	314,000	246,000	1,771,000
Provision	5,825,000	1,250,000
Allowance for loan losses, end of year	35,486,000	29,585,000	28,543,000
Amount of allowance for loan losses for loans deemed to be impaired	589,000	102,000
Amount of allowance for loan losses for loans not deemed to be impaired	34,897,000	29,483,000
Loans:
Total loans, net	2,995,829,000	2,426,119,000
Loans deemed to be impaired	5,379,000	3,252,000	3,051,000
Loans not deemed to be impaired	2,990,450,000	2,422,867,000
Construction and Land Development [Member]
Allowance for Loan Losses:
Allowance for loan losses, beginning of year	331,000	1,092,000
Provision	98,000	(761,000)
Allowance for loan losses, end of year	429,000	331,000	1,092,000
Amount of allowance for loan losses for loans not deemed to be impaired	429,000	331,000
Loans:
Total loans, net	10,909,000	8,992,000
Loans not deemed to be impaired	10,909,000	8,992,000
Commercial and Industrial [Member]
Allowance for Loan Losses:
Allowance for loan losses, beginning of year	11,596,000	10,998,000
Charge-offs	(29,000)	(137,000)
Recoveries	197,000	60,000
Provision	4,949,000	675,000
Allowance for loan losses, end of year	16,713,000	11,596,000	10,998,000
Amount of allowance for loan losses for loans deemed to be impaired	140,000	15,000
Amount of allowance for loan losses for loans not deemed to be impaired	16,573,000	11,581,000
Loans:
Total loans, net	1,314,245,000	812,417,000
Loans deemed to be impaired	439,000	906,000
Loans not deemed to be impaired	1,313,806,000	811,511,000
Municipal [Member]
Allowance for Loan Losses:
Allowance for loan losses, beginning of year	2,566,000	1,838,000
Provision	238,000	728,000
Allowance for loan losses, end of year	2,804,000	2,566,000	1,838,000
Amount of allowance for loan losses for loans not deemed to be impaired	2,804,000	2,566,000
Loans:
Total loans, net	137,607,000	120,455,000
Loans not deemed to be impaired	137,607,000	120,455,000
Commercial Real Estate [Member]
Allowance for Loan Losses:
Allowance for loan losses, beginning of year	11,464,000	10,663,000
Provision	287,000	801,000
Allowance for loan losses, end of year	11,751,000	11,464,000	10,663,000
Amount of allowance for loan losses for loans deemed to be impaired	449,000	87,000
Amount of allowance for loan losses for loans not deemed to be impaired	11,302,000	11,377,000
Loans:
Total loans, net	789,836,000	786,102,000
Loans deemed to be impaired	4,940,000	2,346,000
Loans not deemed to be impaired	784,896,000	783,756,000
Residential Real Estate [Member]
Allowance for Loan Losses:
Allowance for loan losses, beginning of year	2,194,000	2,190,000
Provision	(83,000)	4,000
Allowance for loan losses, end of year	2,111,000	2,194,000	2,190,000
Amount of allowance for loan losses for loans not deemed to be impaired	2,111,000	2,194,000
Loans:
Total loans, net	448,436,000	371,897,000
Loans not deemed to be impaired	448,436,000	371,897,000
Consumer [Member]
Allowance for Loan Losses:
Allowance for loan losses, beginning of year	312,000	365,000
Charge-offs	(209,000)	(295,000)
Recoveries	112,000	186,000
Provision	26,000	56,000
Allowance for loan losses, end of year	241,000	312,000	365,000
Amount of allowance for loan losses for loans not deemed to be impaired	241,000	312,000
Loans:
Total loans, net	20,439,000	21,893,000
Loans not deemed to be impaired	20,439,000	21,893,000
Home Equity [Member]
Allowance for Loan Losses:
Allowance for loan losses, beginning of year	1,065,000	1,111,000
Charge-offs		(22,000)
Recoveries	5,000
Provision	138,000	(24,000)
Allowance for loan losses, end of year	1,208,000	1,065,000	1,111,000
Amount of allowance for loan losses for loans not deemed to be impaired	1,208,000	1,065,000
Loans:
Total loans, net	274,357,000	304,363,000
Loans not deemed to be impaired	274,357,000	304,363,000
Unallocated [Member]
Allowance for Loan Losses:
Allowance for loan losses, beginning of year	57,000	286,000
Provision	172,000	(229,000)
Allowance for loan losses, end of year	229,000	57,000	$ 286,000
Amount of allowance for loan losses for loans not deemed to be impaired	$ 229,000	$ 57,000