Note 18 - Westamerica Bancorporation (Parent Company Only Condensed Financial Information) - Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Net Book Value	$ 27,016	$ 28,819
Other assets	297,310	319,146
Total assets	6,364,592	6,950,317
Other liabilities	59,269	65,125
Total liabilities	5,591,698	6,348,207
Shareholders' equity	772,894	602,110	$ 827,102	$ 844,809
Total liabilities and shareholders' equity	6,364,592	6,950,317
Parent Company [Member]
Cash	154,999	99,478
Net Book Value	9,123	9,411
Other assets	45,694	44,831
Total assets	789,143	618,918
Other liabilities	16,178	16,768
Total liabilities	16,249	16,808
Shareholders' equity	772,894	602,110
Total liabilities and shareholders' equity	789,143	618,918
Parent Company [Member] | Related Party [Member]
Accounts receivable from Westamerica Bank	341	245
Accounts payable to Westamerica Bank	71	40
Parent Company [Member] | Bank Subsidiary [Member]
Investment	578,535	464,500
Parent Company [Member] | Non-bank Subsidiaries [Member]
Investment	$ 451	$ 453