Document And Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2021	Feb. 18, 2022	Jun. 30, 2021
Document Information [Line Items]
Entity Central Index Key	0000311094
Entity Registrant Name	WESTAMERICA BANCORPORATION
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Document Fiscal Period Focus	FY
Document Fiscal Year Focus	2021
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2021
Document Transition Report	false
Entity File Number	001-09383
Entity Incorporation, State or Country Code	CA
Entity Tax Identification Number	94-2156203
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
Title of 12(b) Security	Common Stock, no par value
Trading Symbol	WABC
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Public Float			$ 885,440,863.36
Entity Common Stock, Shares Outstanding		26,867,644
Auditor Name	Crowe LLP
Auditor Location	Sacramento, California
Auditor Firm ID	173
ICFR Auditor Attestation Flag	true