Note 20 - Westamerica Bancorporation (Parent Company Only) (Detail) - Balance Sheets (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Premises and equipment, net	$ 38,639	$ 36,548
Other assets	188,086	226,866
Total assets	4,952,193	5,042,161
Liabilities
Other liabilities	55,113	66,887
Total liabilities	4,392,091	4,483,520
Shareholders’ equity	560,102	558,641	545,287	505,448
Total liabilities and shareholders’ equity	4,952,193	5,042,161
Westamerica Bank [Member] | Parent Company [Member]
Investment in subsidiaries	534,467	538,658
Non-Bank Subsidiaries [Member] | Parent Company [Member]
Investment in subsidiaries	458	459
Parent Company [Member]
Cash	13,219	8,363	1,205	1,200
Money market assets and investment securities available for sale	1,461	2,186
Premises and equipment, net	9,983	11,365
Accounts receivable from Westamerica Bank	613	719
Other assets	30,897	28,485
Total assets	591,098	590,235
Liabilities
Debt financing and notes payable	15,000	15,000
Accounts payable to Westamerica Bank	660	664
Other liabilities	15,336	15,930
Total liabilities	30,996	31,594
Shareholders’ equity	560,102	558,641
Total liabilities and shareholders’ equity	$ 591,098	$ 590,235