Note 9 - Regulatory Capital - Capital Ratios for the Company and the Bank (Details) $ in Thousands		Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Common Equity Tier 1 Capital		$ 841,611	$ 736,414
Common Equity Tier 1 Capital, ratio		0.1876	0.1522
Common Equity Tier 1 Capital Required for Capital Adequacy		$ 313,959	$ 338,748
Common Equity Tier 1 Capital Required for Capital Adequacy, ratio		7.00%	7.00%
Tier 1 Capital		$ 841,611	$ 736,414
Tier 1 Capital, raio		0.1876	0.1522
Tier 1 Capital Required for Capital Adequacy		$ 381,236	$ 411,337
Tier 1 Capital Required for Capital Adequacy, ratio		0.085	0.085
Total Capital		$ 858,680	$ 756,900
Total Capital, ratio		0.1915	0.1564
Total Capital Required for Capital Adequacy		$ 470,939	$ 508,123
Total Capital Required for Capital Adequacy, ratio		0.105	0.105
Leverage Ratio, amount	[1]	$ 841,611	$ 736,414
Leverage Ratio	[1]	0.1286	0.1018
Leverage Ratio Required for Capital Adequacy, amount	[1]	$ 261,739	$ 289,259
Leverage Ratio Required for Capital Adequacy	[1]	0.04	0.04
Westamerica Bank [Member]
Common Equity Tier 1 Capital		$ 641,252	$ 592,804
Common Equity Tier 1 Capital, ratio		0.1446	0.1237
Common Equity Tier 1 Capital Required for Capital Adequacy		$ 310,362	$ 335,351
Common Equity Tier 1 Capital Required for Capital Adequacy, ratio		7.00%	7.00%
Common Equity Tier 1 Capital To Be Well-capitalized		$ 288,193	$ 311,397
Common Equity Tier 1 Capital To Be Well-capitalized, ratio		6.50%	6.50%
Tier 1 Capital		$ 641,252	$ 592,804
Tier 1 Capital, raio		0.1446	0.1237
Tier 1 Capital Required for Capital Adequacy		$ 376,868	$ 407,212
Tier 1 Capital Required for Capital Adequacy, ratio		0.085	0.085
Tier 1 Capital To Be Well-capitalized		$ 354,700	$ 383,258
Tier 1 Capital To Be Well-capitalized, ratio		0.08	0.08
Total Capital		$ 664,321	$ 619,290
Total Capital, ratio		0.1498	0.1293
Total Capital Required for Capital Adequacy		$ 465,543	$ 503,026
Total Capital Required for Capital Adequacy, ratio		0.105	0.105
Total Capital To Be Well-capitalized		$ 443,375	$ 479,073
Total Capital To Be Well-capitalized, ratio		0.10	0.10
Leverage Ratio, amount	[1]	$ 641,252	$ 592,804
Leverage Ratio	[1]	0.0988	0.0826
Leverage Ratio Required for Capital Adequacy, amount	[1]	$ 259,617	$ 287,229
Leverage Ratio Required for Capital Adequacy	[1]	0.04	0.04
Leverage Ratio To Be Well-capitalized, amount	[1]	$ 324,521	$ 359,036
Leverage Ratio To Be Well-capitalized, ratio	[1]	0.05	0.05

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