Note 9 - Fair Value Measurements (Details Textual)	6 Months Ended
Jun. 30, 2019
Securities Selected for OTTI Analysis, Market Price, Threshold	95.00%
Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Appraisal Rate	10.00%

Represents the market price threshold compared to par value for securities selected for other than temporary impairment (OTTI) analysis.

No definition available.

Name:	wabc_SecuritiesSelectedForOTTIAnalysisMarketPriceThreshold
Namespace Prefix:	wabc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration