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The amount of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
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-URI http://asc.fasb.org/extlink&oid=82895541&loc=d3e18845-111554

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Number of financing receivables that have been modified by troubled debt restructurings within the previous 12 months and for which there was a payment default.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
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Reflects the amount of the write-down related to a troubled debt restructuring.

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Data Type:	xbrli:monetaryItemType
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The face amount of loans that will be sold to other entities.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
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Data Type:	xbrli:monetaryItemType
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Recorded investment of consumer mortgage loan receivables secured by residential real estate properties for which formal foreclosure proceedings are in process.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
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-Section 50
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Carrying amount of other real estate not separately disclosed, including, but not limited to, real estate investments, real estate loans that qualify as investments in real estate, and premises that are no longer used in operations may also be included in real estate owned. This does not include real estate assets taken in settlement of troubled loans through surrender or foreclosure.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.10(4))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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The sum of the carrying amounts, as of the date of the latest financial statement presented, of loans receivable which are owned but transferred to serve as collateral for the payment of related debt obligations, primarily secured borrowings and repurchase agreements, and for which the transferees are not permitted to sell or re-pledge them to an unrelated party.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (b)(1)(i)
-URI http://asc.fasb.org/extlink&oid=109249958&loc=SL6224234-111729

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Carrying amount as of the balance sheet date of land and buildings obtained through foreclosure proceedings or defeasance in full or partial satisfaction of a debt arrangement.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5162-111524

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4437-111522

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The amount of the carrying value of financing receivables that have been modified by troubled debt restructurings during the period.

No definition available.

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Data Type:	xbrli:monetaryItemType
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Recorded investment in impaired loan receivables that have been restructured and are on nonaccrual status as of the balance sheet date.

No definition available.

Name:	wabc_ImpairedLoansRestructuredNonaccrualStatus
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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Reflects the unfunded portion of contractual agreements to lend funds to borrowers for loans on nonaccrual status.

No definition available.

Name:	wabc_LoansAndLeasesReceivableNonaccrualCommitmentToLend
Namespace Prefix:	wabc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

The reserve account relating to other real estate.

No definition available.

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

Aggregate amount of purchased loans and leases receivable held in portfolio that may or may not be covered by loss sharing agreements before purchase accounting discounts.

No definition available.

Name:	wabc_PurchasedLoans
Namespace Prefix:	wabc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Number of troubled debt restructurings during period.

No definition available.

Name:	wabc_TroubledDebtRestructuringsDuringPeriod
Namespace Prefix:	wabc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

Name:	srt_MortgageLoansOnRealEstateDescriptionLoanCategoryAxis=wabc_ResidentialRealEstateIncludedInOREOMember
Namespace Prefix:
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Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=wabc_RestructuredLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=wabc_OneLoanMember
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