Revenue Recognition - Summary of Revenue Recognition (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Schedule Of Revenue Recognition [Line Items]
Total net securities brokerage revenue	$ 1,809	$ 1,680	$ 3,479	$ 3,442
Electronic banking fees	5,728	4,984	10,558	9,512
Mortgage banking income	1,670	968	2,776	2,408
Net gain or loss on sale of other real estate owned	229	342	491	307
Total Service Charges on Deposits [Member]
Schedule Of Revenue Recognition [Line Items]
Total trust fees/Total service charges on deposits	5,146		$ 9,969
Trust Account Fees [Member]
Schedule Of Revenue Recognition [Line Items]
Point of revenue recognition			Over time
Total trust fees/Total service charges on deposits	3,558		$ 7,850
WesMark Fees [Member]
Schedule Of Revenue Recognition [Line Items]
Point of revenue recognition			Over time
Total trust fees/Total service charges on deposits	2,194		$ 4,405
Total Trust Fees [Member]
Schedule Of Revenue Recognition [Line Items]
Total trust fees/Total service charges on deposits	5,752	$ 5,572	$ 12,255	$ 11,716
Commercial Banking Fees [Member]
Schedule Of Revenue Recognition [Line Items]
Point of revenue recognition			Over time
Total trust fees/Total service charges on deposits	462		$ 869
Personal Service Charges [Member]
Schedule Of Revenue Recognition [Line Items]
Point of revenue recognition			At a point in time & Over time
Total trust fees/Total service charges on deposits	4,684		$ 9,100
Annuity Commissions [Member]
Schedule Of Revenue Recognition [Line Items]
Point of revenue recognition			At a point in time
Total net securities brokerage revenue	1,310		$ 2,510
Equity and Debt Security Trades [Member]
Schedule Of Revenue Recognition [Line Items]
Point of revenue recognition			At a point in time
Total net securities brokerage revenue	87		$ 189
Managed Money [Member]
Schedule Of Revenue Recognition [Line Items]
Point of revenue recognition			Over time
Total net securities brokerage revenue	163		$ 304
Trail Commissions [Member]
Schedule Of Revenue Recognition [Line Items]
Point of revenue recognition			Over time
Total net securities brokerage revenue	$ 249		$ 476
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
OREO/other assets [Member]
Schedule Of Revenue Recognition [Line Items]
Point of revenue recognition			At a point in time