Note 9 - Regulatory Capital - Capital Ratios for the Company and the Bank (Details) $ in Thousands		Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Common Equity Tier 1 Capital		$ 653,026	$ 604,833
Common Equity Tier 1 Capital, ratio		0.1493	0.1604
Common Equity Tier 1 Capital Required for Capital Adequacy		$ 306,277	$ 263,903
Common Equity Tier 1 Capital Required for Capital Adequacy, ratio		7.00%	7.00%
Tier 1 Capital		$ 653,026	$ 604,833
Tier 1 Capital, raio		0.1493	0.1604
Tier 1 Capital Required for Capital Adequacy		$ 371,908	$ 320,454
Tier 1 Capital Required for Capital Adequacy, ratio		0.0850	0.0850
Total Capital		$ 676,749	$ 628,797
Total Capital, ratio		0.1547	0.1668
Total Capital Required for Capital Adequacy		$ 459,416	$ 395,855
Total Capital Required for Capital Adequacy, ratio		0.1050	0.1050
Leverage Ratio, amount	[1]	$ 653,026	$ 604,833
Leverage Ratio	[1]	0.0906	0.0940
Leverage Ratio Required for Capital Adequacy, amount	[1]	$ 288,423	$ 257,488
Leverage Ratio Required for Capital Adequacy	[1]	0.0400	0.0400
Westamerica Bank [Member]
Common Equity Tier 1 Capital		$ 540,538	$ 484,270
Common Equity Tier 1 Capital, ratio		0.1248	0.1300
Common Equity Tier 1 Capital Required for Capital Adequacy		$ 303,111	$ 260,755
Common Equity Tier 1 Capital Required for Capital Adequacy, ratio		7.00%	7.00%
Common Equity Tier 1 Capital To Be Well-capitalized		$ 281,460	$ 242,130
Common Equity Tier 1 Capital To Be Well-capitalized, ratio		6.50%	6.50%
Tier 1 Capital		$ 540,538	$ 484,270
Tier 1 Capital, raio		0.1248	0.1300
Tier 1 Capital Required for Capital Adequacy		$ 368,063	$ 316,632
Tier 1 Capital Required for Capital Adequacy, ratio		0.0850	0.0850
Tier 1 Capital To Be Well-capitalized		$ 346,412	$ 298,006
Tier 1 Capital To Be Well-capitalized, ratio		0.0800	0.0800
Total Capital		$ 570,260	$ 514,234
Total Capital, ratio		0.1317	0.1380
Total Capital Required for Capital Adequacy		$ 454,666	$ 391,133
Total Capital Required for Capital Adequacy, ratio		0.1050	0.1050
Total Capital To Be Well-capitalized		$ 433,016	$ 372,508
Total Capital To Be Well-capitalized, ratio		0.1000	0.1000
Leverage Ratio, amount	[1]	$ 540,538	$ 484,270
Leverage Ratio	[1]	0.0755	0.0758
Leverage Ratio Required for Capital Adequacy, amount	[1]	$ 286,432	$ 255,560
Leverage Ratio Required for Capital Adequacy	[1]	0.0400	0.0400
Leverage Ratio To Be Well-capitalized, amount	[1]	$ 358,040	$ 319,451
Leverage Ratio To Be Well-capitalized, ratio	[1]	0.0500	0.0500

[1] The leverage ratio consists of Tier 1capital divided by the most recent quarterly average total assets, excluding certain intangible assets.