The current carrying amount of the liability for the freestanding or embedded guarantor's obligations under the guarantee or each group of similar guarantees.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121555522&loc=d3e12069-110248

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Namespace Prefix:	us-gaap_
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Balance Type:	credit
Period Type:	instant

- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

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

- Definition

Amount of loss contingency liability.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121557415&loc=d3e14326-108349

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Balance Type:	credit
Period Type:	instant

- Definition

Amount of increase (decrease) in loss contingency liability.

+ References

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Balance Type:	na
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- Definition

The portion of loan commitments that are not unconditionally cancellable.

+ References

No definition available.

+ Details

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

- Definition

Amount of reserve for unfunded commitments.

+ References

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+ Details

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

- Definition

Reflects the unfunded portion of contractual agreements to lend funds to borrowers.

+ References

No definition available.

+ Details

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

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Note 10 - Commitments and Contingent Liabilities (Details Textual) - USD ($) $ in Thousands	3 Months Ended
	Jun. 30, 2019	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Dec. 31, 2017
Loan Commitments, Not Unconditionally Cancellable		$ 14,947
Unfunded Loan Commitments		264,310		$ 265,311
Guarantor Obligations, Current Carrying Value		580		550
Reserve for Unfunded Commitments			$ 2,160
Other Liabilities [Member]
Reserve for Unfunded Commitments		53		2,160
Financial Standby Letter of Credit [Member]
Letters of Credit Outstanding, Amount		2,441		3,099
Commercial Standby Letters of Credit [Member]
Letters of Credit Outstanding, Amount		$ 0		$ 0
Customer Refunds [Member]
Loss Contingency Accrual, Ending Balance	$ 5,843				$ 5,542
Loss Contingency Accrual, Period Increase (Decrease), Total	$ 301