Note 18 - Westamerica Bancorporation (Parent Company Only Condensed Financial Information) - Cash Flows Statement (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
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
Depreciation and amortization													11,774	16,565	16,617
Other assets													(3,443)	(2,415)	(3,534)
Stock option compensation expense													1,356	1,309	1,419
Provision for deferred income tax													(1,995)	697	2,123
(Decrease) increase in other liabilities													(7,484)	(6,827)	(6,940)
Net Cash Provided by Operating Activities													158,208	113,702	88,735
Net Cash Used in Investing Activities													547,274	(631,286)	(306,084)
Financing Activities:
Exercise of stock options													950	2,255	3,017
Retirement of common stock													(13,747)	(218)	(232)
Common stock dividends paid													45,954	45,182	44,304
Net Cash Used in Financing Activities													(809,404)	(320,265)	728,159
Net change in cash and due from banks													(103,922)	(837,849)	510,810
Cash and Due from Banks at Beginning of Period				294,236				1,132,085				621,275	294,236	1,132,085	621,275
Cash and Due from Banks at End of Period	190,314				294,236				1,132,085				190,314	294,236	1,132,085
Supplemental disclosure of cash flow activities:
Income tax payments for the period													64,017	39,840	27,673
Parent Company [Member]
Net Income													161,768	122,034	86,509
Depreciation and amortization													539	563	569
Decrease (increase) in accounts receivable from affiliates													1,224	(771)	117
Other assets													(2,048)	(1,639)	(1,223)
Stock option compensation expense													1,356	1,309	1,419
Provision for deferred income tax													1,179	881	645
(Decrease) increase in other liabilities													(1,326)	(38)	254
Earnings of subsidiaries greater than subsidiary dividends													(48,211)	(49,654)	(55,114)
Net Cash Provided by Operating Activities													114,481	72,685	33,176
Purchases of equipment													(209)	(5)	(78)
Net Cash Used in Investing Activities													(209)	(5)	(78)
Financing Activities:
Exercise of stock options													950	2,255	3,017
Retirement of common stock													(13,747)	(218)	(232)
Common stock dividends paid													45,954	45,182	44,304
Net Cash Used in Financing Activities													(58,751)	(43,145)	(41,519)
Net change in cash and due from banks													55,521	29,535	(8,421)
Cash and Due from Banks at Beginning of Period				$ 99,478				$ 69,943				$ 78,364	99,478	69,943	78,364
Cash and Due from Banks at End of Period	$ 154,999				$ 99,478				$ 69,943				154,999	99,478	69,943
Supplemental disclosure of cash flow activities:
Income tax payments for the period													$ 64,017	$ 39,840	$ 27,673