Note 20 - Westamerica Bancorporation (Parent Company Only) (Detail) - Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Operating Activities
Net income	$ 81,127	$ 87,888	$ 94,577
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	14,074	14,253	15,327
Increase in other assets	142	2,773	23,404
Stock option compensation expense	1,450	1,425	1,380
Tax benefit decrease (increase) upon exercise of stock options	119	248	(1,004)
(Refund) provision for deferred income tax	(8,394)	1,213	(10,761)
Increase in other liabilities	17,147	431	(16,767)
Net cash provided by operating activities	119,464	120,393	114,970
Investing Activities
Purchases of premises and equipment	(4,834)	(3,309)	(1,448)
Net cash used in investing activities	5,697	67,645	298,515
Financing Activities
Net change in short-term debt	(62,001)	(16,868)	(205,819)
Exercise of stock options/issuance of shares	7,635	14,374	16,688
Tax benefit (decrease) increase upon exercise of stock options	(119)	(248)	1,004
Retirement of common stock including repurchases	(51,499)	(60,505)	(28,719)
Dividends	(41,005)	(41,670)	(42,094)
Net cash used in financing activities	(163,824)	3,214	(435,827)
Net change in cash	(38,663)	191,252	(22,342)
Supplemental disclosure of cash flow activity:
Interest paid for the period	6,814	11,271	15,414
Income tax payments for the period	34,111	28,826	50,388
Parent Company [Member]
Operating Activities
Net income	81,127	87,888	94,577
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	297	126	172
Decrease (increase) in accounts receivable from affiliates	105	(18)	(11)
Increase in other assets	(1,960)	(1,951)	(2,212)
Stock option compensation expense	1,450	1,425	1,380
Tax benefit decrease (increase) upon exercise of stock options	119	248	(1,004)
(Refund) provision for deferred income tax	(1,306)	963	789
Increase in other liabilities	1,182	217	1,833
Earnings of subsidiaries less (greater) than subsidiary dividends	7,746	17,523	(27,896)
Writedown of property and equipment	1,504	599	228
Net cash provided by operating activities	90,264	107,020	67,856
Investing Activities
Purchases of premises and equipment	(420)	(1,154)	(30)
Net decrease in short term investments		341
Net cash used in investing activities	(420)	(813)	(30)
Financing Activities
Net change in short-term debt		(1,000)	(14,700)
Net reductions in notes payable and long-term borrowings		(10,000)
Exercise of stock options/issuance of shares	7,635	14,374	16,688
Tax benefit (decrease) increase upon exercise of stock options	(119)	(248)	1,004
Retirement of common stock including repurchases	(51,499)	(60,505)	(28,719)
Dividends	(41,005)	(41,670)	(42,094)
Net cash used in financing activities	(84,988)	(99,049)	(67,821)
Net change in cash	4,856	7,158	5
Cash at beginning of year	8,363	1,205	1,200
Cash at end of year	13,219	8,363	1,205
Supplemental disclosure of cash flow activity:
Interest paid for the period	1,105	1,794	1,824
Income tax payments for the period	$ 34,111	$ 28,826	$ 50,388