Note 20 - Westamerica Bancorporation (Parent Company Only) (Detail) - Statements of Income and Comprehensive Income (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Salaries and benefits	$ 57,388	$ 58,501	$ 61,748
Income before taxes and equity in undistributed income of subsidiaries	106,557	120,816	131,422
Income tax benefit	(25,430)	(32,928)	(36,845)
Net income	81,127	87,888	94,577
Other comprehensive income (loss), net of tax	3,256	11,210	(3,555)
Comprehensive income	84,383	99,098	91,022
Parent Company [Member]
Dividends from subsidiaries	88,755	106,756	68,784
Interest income	8	11	11
Other income	7,907	7,780	7,262
Total income	96,670	114,547	76,057
Interest on borrowings	820	859	1,824
Salaries and benefits	7,090	6,620	7,219
Other expense	1,734	2,356	1,749
Total expenses	9,644	9,835	10,792
Income before taxes and equity in undistributed income of subsidiaries	87,026	104,712	65,265
Income tax benefit	1,847	699	1,416
Earnings of subsidiaries (less) greater than subsidiary dividends	(7,746)	(17,523)	27,896
Net income	81,127	87,888	94,577
Other comprehensive income (loss), net of tax	3,256	11,210	(3,555)
Comprehensive income	$ 84,383	$ 99,098	$ 91,022