Note 20 - Condensed Financial Information of Cathay General Bancorp (Detail) - Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Cash flows from Operating Activities
Net income/(loss)	$ 117,438	$ 100,150	$ 11,565
Write-downs on venture capital and other investments	309	379	515
Write-downs on impaired securities	181	200	492
Excess tax short-fall from stock options	620	290	539
(Decrease)/increase in other assets	37,093	(534)	34,594
Increase/(decrease) in other liabilities	(2,256)	3,746	(13,368)
Net cash used in operating activities	164,576	174,050	168,505
Cash flows from Investment Activities
Proceeds from sale/(purchase) of available-for-sale securities	(517,513)	(571,093)	(3,366,780)
Net cash provided by/(used in) investment activities	(46,536)	127,050	596,595
Cash flows from Financing Activities
Issuance of common stock			124,928
Proceeds from exercise of stock options	764	1,306
Excess tax short-fall from share-based payment arrangements	(620)	(290)	(539)
Net cash (used in)/provided by financing activities	(91,019)	(270,559)	(777,877)
Increase in cash and cash equivalents	27,021	30,541	(12,777)
Cash and cash equivalents,	144,909	117,888	87,347
Parent Company [Member] | Beginning of Period [Member]
Cash flows from Financing Activities
Cash and cash equivalents,	809	787	448
Parent Company [Member] | End of Period [Member]
Cash flows from Financing Activities
Cash and cash equivalents,	639	809	787
Parent Company [Member]
Cash flows from Operating Activities
Net income/(loss)	117,438	100,150	11,565
Dividends in excess of earnings of subsidiaries	36,463
Equity in undistributed earnings of subsidiaries	36,463	(102,492)	(14,427)
Gains on sale of securities	(3,380)
Increase in accrued expense	12	24	3
Write-downs on venture capital and other investments	262	321	521
Write-downs on impaired securities	181	200	492
Loss/(gains) in fair value of warrants	114	(215)	29
Excess tax short-fall from stock options	620	290	539
(Decrease)/increase in other assets	1,820	(121)	1,040
Increase/(decrease) in other liabilities	59	(221)	(607)
Net cash used in operating activities	153,589	(2,064)	(845)
Cash flows from Investment Activities
Additional investment in subsidiary			(94,000)
Decrease/(increase) in short-term investment	(142,300)	17,500	(12,000)
Proceeds from sale/(purchase) of available-for-sale securities	4,849		(418)
Venture capital investments	(694)	(671)	(1,056)
Net cash provided by/(used in) investment activities	(138,145)	16,829	(107,474)
Cash flows from Financing Activities
Cash dividends	(16,049)	(16,046)	(16,041)
Issuance of common stock			124,928
Proceeds from shares issued under the Dividend Reinvestment Plan	291	287	310
Proceeds from exercise of stock options	764	1,306
Excess tax short-fall from share-based payment arrangements	(620)	(290)	(539)
Net cash (used in)/provided by financing activities	(15,614)	(14,743)	108,658
Increase in cash and cash equivalents	(170)	22	339
Cash and cash equivalents,	$ 639	$ 809