Note 5 - Concentration of Credit Risk (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Loan Commitments And Standby Letters Of Credit (in Dollars)	$ 67,431	$ 69,345
Commercial Real Estate Loans [Member]
Loan To Value Ratio Requirement	75.00%
Residential Real Estate Loans [Member]
Loan To Value Ratio Requirement	80.00%

Reflects the unfunded portion of contractual agreements to lend funds to borrowers and the total amount of the contingent obligation under letters of credit outstanding related to real estate loans as of the reporting date.

No definition available.

Name:	wabc_LoanCommitmentsAndStandbyLettersOfCredit
Namespace Prefix:	wabc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant