Note 18 - Westamerica Bancorporation (Parent Company Only Condensed Financial Information) - Statements of Income and Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Salaries and related benefits													$ 47,871	$ 46,125	$ 48,011
Income before taxes and equity in undistributed income of subsidiaries	$ 54,848	$ 56,946	$ 54,743	$ 55,042	$ 54,096	$ 47,366	$ 34,149	$ 29,980	$ 29,467	$ 29,903	$ 30,533	$ 27,124	221,579	165,591	117,027
Income tax expense													(59,811)	(43,557)	(30,518)
Net Income	$ 39,468	$ 41,601	$ 40,248	$ 40,451	$ 39,344	$ 34,760	$ 25,314	$ 22,616	$ 21,720	$ 22,063	$ 22,579	$ 20,147	161,768	122,034	86,509
Other comprehensive income (loss), net of tax													65,823	(305,769)	(64,748)
Comprehensive income (loss)													227,591	(183,735)	21,761
Parent Company [Member]
Dividends from subsidiaries													110,769	70,267	29,279
Interest income													57	49	44
Other income													11,935	11,386	11,608
Total income													122,761	81,702	40,931
Salaries and related benefits													5,710	5,832	6,612
Other expense													2,315	2,609	2,279
Total expense													8,025	8,441	8,891
Income before taxes and equity in undistributed income of subsidiaries													114,736	73,261	32,040
Income tax expense													(1,179)	(881)	(645)
Earnings of subsidiaries greater than subsidiary dividends													48,211	49,654	55,114
Net Income													161,768	122,034	86,509
Other comprehensive income (loss), net of tax													65,823	(305,769)	(64,748)
Comprehensive income (loss)													$ 227,591	$ (183,735)	$ 21,761