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Note 8 - Deposits and Borrowed Funds (Details Textual) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024
Demand Deposit Overdrafts	$ 490,000		$ 490,000		$ 591,000
Interest Expense, Time Deposits, $100,000 or More	12,000	$ 30,000	43,000	$ 71,000
Short-term Line of Credit	0		0
Federal Reserve [Member]
Line of Credit Facility, Maximum Borrowing Capacity	647,776,000		647,776,000
Short-term Line of Credit	0		0
Federal Reserve [Member] | Assets Available to be Pledged as Collateral [Member]
Debt Securities, Available-for-Sale and Held-to-Maturity, after Allowance for Credit Loss	1,970,022		1,970,022
Corresponding Banks [Member]
Line of Credit Facility, Maximum Borrowing Capacity	$ 60,000,000		$ 60,000,000

X

- Definition

Amount, after allowance for credit loss, of investment in debt security measured at amortized cost (held-to-maturity) and investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481830/320-10-45-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1

+ Details

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

X

- Definition

Interest expense incurred on time deposits, including certificates of deposits, of $100,000 or more in domestic offices.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 942
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(6))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478524/942-220-S99-1

+ Details

Name:	us-gaap_InterestExpenseTimeDeposits100000OrMore
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22)(b))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(19)(b))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Amounts honored in excess of customers account balances, recorded by the bank as a loan.

+ References

No definition available.

+ Details

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

X

- Definition

The amount of short-term line of credit outstanding.

+ References

No definition available.

+ Details

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

X

- Details

Name:	us-gaap_LineOfCreditFacilityAxis=wabc_FederalReserveMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_PledgedStatusAxis=wabc_AssetsAvailableToBePledgedAsCollateralMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_LineOfCreditFacilityAxis=wabc_CorrespondingBanksMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: