Note 4 - Loans and Allowance for Credit Losses (Details) - Changes in the Accretable Yield for Purchased Loans (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
Sep. 30, 2014	Dec. 31, 2013
Changes in the Accretable Yield for Purchased Loans [Abstract]
Balance at the beginning of the period	$ 2,505	$ 4,948
Reclassification from nonaccretable difference	3,513	12,504
Accretion	(3,287)	(14,947)
Reduction in FDIC indemnification asset	278	11,438
(Increase) in interest income	(3,009)	(3,509)
Balance at the end of the period	$ 2,731	$ 2,505

Amount of the excess of a loan's cash flows expected to be collected over the investor's initial investment in acquired loans not accounted for as debt securities with evidence of deterioration of credit quality.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 30

-Section 15

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=27045933&loc=d3e13503-111538

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 30

-Section 50

-Paragraph 2

-Subparagraph (a)(2)

-URI http://asc.fasb.org/extlink&oid=6379141&loc=d3e15032-111544

+ Details

Name:	us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAccretableYield
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of decrease from accretion of the excess of a loan's cash flows expected to be collected over the investor's initial investment in acquired loans not accounted for as debt securities with evidence of deterioration of credit quality.

+ References

+ Details

Name:	us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAccretableYieldAccretion
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of increase of reclassification from nonaccretable difference to the excess of a loan's cash flows expected to be collected over the investor's initial investment in acquired loans not accounted for as debt securities with evidence of deterioration of credit quality.

+ References

+ Details

Name:	us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAccretableYieldReclassificationsFromNonaccretableDifference
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The current period adjustment to accretion for FDIC indemnification offset. This relates to loans not accounted for as debt securities, with evidence of deterioration of credit quality since origination that were acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.

+ References

No definition available.

+ Details

Name:	wabc_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAccretableYieldAccretionReductionInFDICIndemnificationAsset
Namespace Prefix:	wabc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The current period net recognition due to accretion of the excess of a loan's cash flows expected to be collected over the investor's initial investment in the loan, less the current period adjustment related to FDIC indemnification offset. This relates to loans not accounted for as debt securities, with evidence of deterioration of credit quality since origination that were acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.

+ References

No definition available.

+ Details

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

- Details

Name:	wabc_ChangesInTheAccretableYieldForPurchasedLoansAbstract
Namespace Prefix:	wabc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration