Note 7 - Deposits and Borrowed Funds - Borrowed Funds, Highest Month End Balances (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020
Securities Sold under Agreements to Repurchase [Member]
Securities sold under repurchase agreements	$ 146,552	$ 110,846

For borrowed funds, maximum month-end borrowings during the period.

No definition available.

Name:	wabc_BorrowedFundsMaximumMonthEndOutstandingAmount
Namespace Prefix:	wabc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration