Revenue Recognition - Summary of Revenue Recognition (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Schedule Of Revenue Recognition [Line Items]
Total net securities brokerage revenue	$ 6,189	$ 6,990	$ 7,186
Debit card sponsorship income	2,792	328
Electronic banking fees	17,524	22,634	23,300
Mortgage banking income	22,736	8,219	5,840
Net gains on other real estate owned and other assets	103	732	524
Total Service Charges on Deposits [Member]
Schedule Of Revenue Recognition [Line Items]
Total trust fees/Total service charges on deposits	21,943	26,974	23,670
Trust Account fees [Member]
Schedule Of Revenue Recognition [Line Items]
Total trust fees/Total service charges on deposits	17,753	$ 18,059	15,833
Point of revenue recognition		Over time
WesMark Fees [Member]
Schedule Of Revenue Recognition [Line Items]
Total trust fees/Total service charges on deposits	8,582	$ 8,520	8,790
Point of revenue recognition		Over time
Total Trust Fees [Member]
Schedule Of Revenue Recognition [Line Items]
Total trust fees/Total service charges on deposits	26,335	$ 26,579	24,623
Commercial Banking Fees [Member]
Schedule Of Revenue Recognition [Line Items]
Total trust fees/Total service charges on deposits	2,337	$ 2,033	1,733
Point of revenue recognition		Over time
Personal Service Charges [Member]
Schedule Of Revenue Recognition [Line Items]
Total trust fees/Total service charges on deposits	19,606	$ 24,941	21,937
Point of revenue recognition		At a point in time and over time
Annuity Commissions [Member]
Schedule Of Revenue Recognition [Line Items]
Total net securities brokerage revenue	3,906	$ 4,829	5,178
Point of revenue recognition		At a point in time
Equity And Debt Security Trades [Member]
Schedule Of Revenue Recognition [Line Items]
Total net securities brokerage revenue	349	$ 434	429
Point of revenue recognition		At a point in time
Managed Money [Member]
Schedule Of Revenue Recognition [Line Items]
Total net securities brokerage revenue	952	$ 738	647
Point of revenue recognition		Over time
Trail Commissions [Member]
Schedule Of Revenue Recognition [Line Items]
Total net securities brokerage revenue	982	$ 989	932
Point of revenue recognition		Over time
Payment Processing Fees [Member]
Schedule Of Revenue Recognition [Line Items]
Payment Processing Fees	$ 3,010	$ 3,002	$ 1,028
Point of revenue recognition		At a point in time and over time
Debit Card Sponsorship Income [Member]
Schedule Of Revenue Recognition [Line Items]
Point of revenue recognition		At a point in time and over time
Electronic Banking Fees [Member]
Schedule Of Revenue Recognition [Line Items]
Point of revenue recognition		At a point in time
Mortgage [Member]
Schedule Of Revenue Recognition [Line Items]
Point of revenue recognition		At a point in time
Other Real Estate Owned and Other Assets [Member]
Schedule Of Revenue Recognition [Line Items]
Point of revenue recognition		At a point in time