Revenue Recognition - Summary of Revenue Recognition (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Schedule of Revenue Recognition [Line Items]
Total net securities brokerage revenue	$ 1,973	$ 1,809	$ 3,833	$ 3,479
Electronic banking fees	7,154	5,728	13,046	10,558
Mortgage banking income	1,618	1,670	2,674	2,776
Net gain on other real estate owned and other assets	376	229	512	491
Total Service Charges on Deposits [Member]
Schedule of Revenue Recognition [Line Items]
Total trust fees/Total service charges on deposits	6,197	5,146	12,747	9,969
Trust Account fees [Member]
Schedule of Revenue Recognition [Line Items]
Total trust fees/Total service charges on deposits	4,208	3,558	$ 9,263	7,850
Point of revenue recognition			Over time
WesMark Fees [Member]
Schedule of Revenue Recognition [Line Items]
Total trust fees/Total service charges on deposits	2,131	2,194	$ 4,191	4,405
Point of revenue recognition			Over time
Total Trust Fees [Member]
Schedule of Revenue Recognition [Line Items]
Total trust fees/Total service charges on deposits	6,339	5,752	$ 13,454	12,255
Commercial Banking Fees [Member]
Schedule of Revenue Recognition [Line Items]
Total trust fees/Total service charges on deposits	499	462	$ 967	869
Point of revenue recognition			Over time
Personal Service Charges [Member]
Schedule of Revenue Recognition [Line Items]
Total trust fees/Total service charges on deposits	5,698	4,684	$ 11,780	9,100
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	1,454	1,310	$ 2,815	2,510
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	103	87	$ 206	189
Point of revenue recognition			At a point in time
Managed Money [Member]
Schedule of Revenue Recognition [Line Items]
Total net securities brokerage revenue	174	163	$ 331	304
Point of revenue recognition			Over time
Trail Commissions [Member]
Schedule of Revenue Recognition [Line Items]
Total net securities brokerage revenue	242	$ 249	$ 481	$ 476
Point of revenue recognition			Over time
Payment Processing Fees [Member]
Schedule of Revenue Recognition [Line Items]
Payment Processing Fees	$ 785		$ 1,433
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