Note 18 - Westamerica Bancorporation (Parent Company Only Condensed Financial Information) - Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Premises and equipment, net	$ 31,155	$ 32,813
Other assets	185,415	159,903
Total assets	7,461,026	6,747,931
Other liabilities	73,722	112,598
Total liabilities	6,633,924	5,903,122
Shareholders' equity	827,102	844,809	$ 731,417	$ 615,591
Total liabilities and shareholders' equity	7,461,026	6,747,931
Parent Company [Member]
Cash	69,943	78,364	$ 122,663	$ 72,878
Premises and equipment, net	9,968	10,459
Accounts receivable from Westamerica Bank	224	257
Other assets	42,026	40,852
Total assets	843,229	860,635
Accounts payable to Westamerica Bank	62	29
Other liabilities	16,065	15,797
Total liabilities	16,127	15,826
Shareholders' equity	827,102	844,809
Total liabilities and shareholders' equity	843,229	860,635
Parent Company [Member] | Bank Subsidiary [Member]
Investment	720,614	730,248
Parent Company [Member] | Non-bank Subsidiaries [Member]
Investment	$ 454	$ 455