Document And Entity Information - shares	6 Months Ended
	Jun. 30, 2019	Jul. 29, 2019
Document Information [Line Items]
Entity Central Index Key	0000311094
Entity Shell Company	false
Entity Registrant Name	WESTAMERICA BANCORPORATION
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Document Fiscal Period Focus	Q2
Document Fiscal Year Focus	2019
Entity Small Business	false
Entity Emerging Growth Company	false
Document Quarterly Report	true
Document Transition Report	false
Document Type	10-Q
Document Period End Date	Jun. 30, 2019
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
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Title of 12(b) Security	Common Stock
Trading Symbol	WABC
Security Exchange Name	NASDAQ
Entity Common Stock, Shares Outstanding		26,981,750