Amount of allowance for credit loss on financing receivable. Excludes allowance for financing receivable covered under loss sharing agreement.

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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

+ References

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- Definition

Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

+ References

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

- Definition

The balance of financing receivables that were collectively evaluated for impairment.

+ References

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- Definition

The balance of financing receivables that were individually evaluated for impairment.

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- References

No definition available.

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- References

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- Definition

Aggregate amount of covered and noncovered loans and leases receivable held in portfolio before allowance and net of deferred fees, costs and purchase accounting discounts.

+ References

No definition available.

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Note 4 - Loans, Allowance for Loan Losses and Other Real Estate Owned - Recorded Investment in Loans Evaluated for Impairment (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
wabc_AllowanceForLoanLossesAbstract
Individually evaluated for impairment	$ 2,587		$ 2,752
Collectively evaluated for impairment	17,530		18,599
Total	20,117	$ 20,477	21,351	$ 23,040	$ 23,081	$ 23,009
wabc_CarryingValueOfLoansAbstract
Individually evaluated for impairment	16,171		19,242
Collectively evaluated for impairment	1,145,541		1,187,960
Total	1,161,712		1,207,202
Unallocated Financing Receivable [Member]
wabc_AllowanceForLoanLossesAbstract
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	4,052		3,177
Total	4,052	3,449	3,177	3,759	3,918	3,666
wabc_CarryingValueOfLoansAbstract
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	0		0
Total	0		0
Commercial Portfolio Segment [Member]
wabc_AllowanceForLoanLossesAbstract
Individually evaluated for impairment	2,587		2,752
Collectively evaluated for impairment	2,648		3,559
Total	5,235	6,506	6,311	8,275	8,517	7,746
wabc_CarryingValueOfLoansAbstract
Individually evaluated for impairment	9,368		9,944
Collectively evaluated for impairment	234,209		265,136
Total	243,577		275,080
Commercial Real Estate Portfolio Segment [Member]
wabc_AllowanceForLoanLossesAbstract
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	4,057		3,884
Total	4,057	3,927	3,884	3,789	3,824	3,849
wabc_CarryingValueOfLoansAbstract
Individually evaluated for impairment	6,531		8,438
Collectively evaluated for impairment	571,134		572,042
Total	577,665		580,480
Construction Portfolio Segment [Member]
wabc_AllowanceForLoanLossesAbstract
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	1,117		1,465
Total	1,117	853	1,465	210	175	335
wabc_CarryingValueOfLoansAbstract
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	5,482		3,982
Total	5,482		3,982
Residential Portfolio Segment [Member]
wabc_AllowanceForLoanLossesAbstract
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	238		869
Total	238	261	869	1,064	908	995
wabc_CarryingValueOfLoansAbstract
Individually evaluated for impairment	195		717
Collectively evaluated for impairment	37,618		44,149
Total	37,813		44,866
Consumer Portfolio Segment [Member]
wabc_AllowanceForLoanLossesAbstract
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	5,418		5,645
Total	5,418	$ 5,481	5,645	$ 5,943	$ 5,739	$ 6,418
wabc_CarryingValueOfLoansAbstract
Individually evaluated for impairment	77		143
Collectively evaluated for impairment	297,098		302,651
Total	$ 297,175		$ 302,794