Note 19 - Westamerica Bancorporation (Parent Company Only) (Details) - Statements of Income and Comprehensive Income (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Note 19 - Westamerica Bancorporation (Parent Company Only) (Details) - Statements of Income and Comprehensive Income [Line Items]
Salaries and benefits	$ 56,633	$ 57,388	$ 58,501
Income before taxes and equity in undistributed income of subsidiaries	86,122	106,557	120,816
Income tax benefit	(18,945)	(25,430)	(32,928)
Net income	67,177	81,127	87,888
Other comprehensive (loss) income, net of tax	(10,312)	3,256	11,210
Comprehensive income	56,865	84,383	99,098
Parent Company [Member]
Note 19 - Westamerica Bancorporation (Parent Company Only) (Details) - Statements of Income and Comprehensive Income [Line Items]
Dividends from subsidiaries	88,754	88,755	106,756
Interest income	14	8	11
Other income	8,684	7,907	7,780
Total income	97,452	96,670	114,547
Interest on borrowings	707	820	859
Salaries and benefits	7,120	7,090	6,620
Other expense	2,174	1,734	2,356
Total expenses	10,001	9,644	9,835
Income before taxes and equity in undistributed income of subsidiaries	87,451	87,026	104,712
Income tax benefit	732	1,847	699
Earnings of subsidiaries less than subsidiary dividends	(21,006)	(7,746)	(17,523)
Net income	67,177	81,127	87,888
Other comprehensive (loss) income, net of tax	(10,312)	3,256	11,210
Comprehensive income	$ 56,865	$ 84,383	$ 99,098