Revenue Recognition - Summary of Revenue Recognition (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Schedule of Revenue Recognition [Line Items]
Total net securities brokerage revenue	$ 1,765	$ 1,836	$ 5,597	$ 5,315
Electronic banking fees	5,253	6,139	18,299	16,697
Mortgage banking income	2,588	1,521	5,262	4,297
Net gain on other real estate owned and other assets	158	150	670	641
Total Service Charges on Deposits [Member]
Schedule of Revenue Recognition [Line Items]
Total trust fees/Total service charges on deposits	7,056	6,313	19,803	16,282
Trust Account fees [Member]
Schedule of Revenue Recognition [Line Items]
Total trust fees/Total service charges on deposits	4,265	4,006	$ 13,529	11,856
Point of revenue recognition			Over time
WesMark Fees [Member]
Schedule of Revenue Recognition [Line Items]
Total trust fees/Total service charges on deposits	2,160	2,259	$ 6,351	6,664
Point of revenue recognition			Over time
Total Trust Fees [Member]
Schedule of Revenue Recognition [Line Items]
Total trust fees/Total service charges on deposits	6,425	6,265	$ 19,880	18,520
Commercial Banking Fees [Member]
Schedule of Revenue Recognition [Line Items]
Total trust fees/Total service charges on deposits	513	563	$ 1,480	1,406
Point of revenue recognition			Over time
Personal Service Charges [Member]
Schedule of Revenue Recognition [Line Items]
Total trust fees/Total service charges on deposits	6,543	5,750	$ 18,323	14,876
Point of revenue recognition			At a point in time & over time
Annuity Commissions [Member]
Schedule of Revenue Recognition [Line Items]
Total net securities brokerage revenue	1,204	1,326	$ 4,018	3,836
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	151	116	$ 356	304
Point of revenue recognition			At a point in time
Managed Money [Member]
Schedule of Revenue Recognition [Line Items]
Total net securities brokerage revenue	153	173	$ 484	477
Point of revenue recognition			Over time
Trail Commissions [Member]
Schedule of Revenue Recognition [Line Items]
Total net securities brokerage revenue	257	221	$ 739	698
Point of revenue recognition			Over time
Payment Processing Fees [Member]
Schedule of Revenue Recognition [Line Items]
Payment Processing Fees	$ 709	$ 311	$ 2,142	$ 311
Point of revenue recognition			At a point in time & 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