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, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Salaries and benefits													$ 48,011	$ 50,749	$ 51,054
Income before taxes and equity in undistributed income of subsidiaries	$ 29,467	$ 29,903	$ 30,533	$ 27,124	$ 31,894	$ 26,772	$ 25,904	$ 22,233	$ 26,950	$ 27,016	$ 25,866	$ 25,385	117,027	106,803	105,216
Income tax (expense) benefit													(30,518)	(26,390)	(24,827)
Net income	$ 21,720	$ 22,063	$ 22,579	$ 20,147	$ 23,838	$ 20,051	$ 19,562	$ 16,962	$ 20,728	$ 20,390	$ 19,625	$ 19,646	86,509	80,413	80,389
Other comprehensive (loss) income, net of tax													(64,748)	88,361	66,047
Comprehensive income													21,761	168,774	146,436
Parent Company [Member]
Dividends from subsidiaries													29,279	10,783	80,067
Interest income													44	56	54
Other income													11,608	11,438	8,778
Total income													40,931	22,277	88,899
Salaries and benefits													6,612	7,107	6,978
Other expense													2,279	2,206	3,729
Total expense													8,891	9,313	10,707
Income before taxes and equity in undistributed income of subsidiaries													32,040	12,964	78,192
Income tax (expense) benefit													(645)	(454)	636
Earnings of subsidiaries greater than subsidiary dividends													55,114	67,903	1,561
Net income													86,509	80,413	80,389
Other comprehensive (loss) income, net of tax													(64,748)	88,361	66,047
Comprehensive income													$ 21,761	$ 168,774	$ 146,436