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, 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
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
Depreciation and amortization													16,617	22,647	20,626
Increase in other assets													(3,534)	(3,528)	(14,806)
Stock option compensation expense													1,419	1,875	1,744
Provision (benefit) for deferred income tax													2,123	(246)	3,393
Increase in other liabilities													(6,940)	14,280	(5,298)
Gain on disposal of premises and equipment													0	(71)	0
Net Cash Provided by Operating Activities													88,735	107,744	80,702
Purchases of equipment													(78)	0	0
Net Cash Used by Investing Activities													(306,084)	(748,922)	(22,297)
Financing Activities
Exercise of stock options													3,017	2,838	13,699
Retirement of common stock													(232)	(16,496)	(488)
Common stock dividends paid													44,304	44,285	43,942
Net Cash Used in Financing Activities													728,159	889,032	(105,268)
Interest paid for the period													2,027	1,830	1,898
Income tax payments for the period													27,673	26,462	24,491
Parent Company [Member]
Net income													86,509	80,413	80,389
Depreciation and amortization													569	608	449
Decrease (increase) in accounts receivable from affiliates													117	(150)	80
Increase in other assets													(1,223)	(2,421)	(71)
Stock option compensation expense													1,419	1,875	1,744
Provision (benefit) for deferred income tax													645	428	(315)
Increase in other liabilities													254	855	856
Earnings of subsidiaries greater than subsidiary dividends													(55,114)	(67,903)	(1,561)
Gain on disposal of premises and equipment													0	(61)	(1,055)
Net Cash Provided by Operating Activities													33,176	13,644	80,516
Net Cash Used by Investing Activities													(78)	0	0
Financing Activities
Exercise of stock options													3,017	2,838	13,699
Retirement of common stock													(232)	(16,496)	(488)
Common stock dividends paid													44,304	44,285	43,942
Net Cash Used in Financing Activities													(41,519)	(57,943)	(30,731)
Net change in cash and due from banks													(8,421)	(44,299)	49,785
Cash and due from banks at beginning of period				$ 78,364				$ 122,663				$ 72,878	78,364	122,663	72,878
Cash and due from banks at end of period	$ 69,943				$ 78,364				$ 122,663				69,943	78,364	122,663
Interest paid for the period													0	0	0
Income tax payments for the period													$ 27,673	$ 26,462	$ 24,491