| Cover | Apr. 16, 2020 |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Apr. 16, 2020 |
| Entity File Number | 001-09383 |
| Entity Registrant Name | WESTAMERICA BANCORPORATION |
| Entity Central Index Key | 0000311094 |
| Entity Tax Identification Number | 94-2156203 |
| Entity Incorporation, State or Country Code | CA |
| Entity Address, Address Line One | 1108 Fifth Avenue |
| Entity Address, City or Town | San Rafael |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 94901 |
| City Area Code | 707 |
| Local Phone Number | 863-6000 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, no par value |
| Trading Symbol | WABC |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
