Note 20 - Condensed Financial Information of Cathay General Bancorp - Statements of Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash Flows from Operating Activities
Net income	$ 67,377	$ 72,835	$ 72,244	$ 66,679	$ 64,648	$ 69,757	$ 73,664	$ 63,816	$ 279,135	$ 271,885	$ 176,042
Write-downs on venture capital and other investments									39,898	40,871	4,572
Net change in other assets									6,163	(13,984)	25,747
Gain from acquisition									0	(340)	(5,628)
Net change in other liabilities									21,061	13,848	(2,641)
Net cash provided by operating activities									434,979	336,538	248,874
Cash Flows from Investing Activities
Purchase of available-for-sale securities									(770,206)	(497,787)	(607,574)
Acquisitions, net of cash acquired									0	0	(1,392)
Net cash provided by/(used in) investment activities									(1,381,674)	(1,134,327)	(903,987)
Cash Flows from Financing Activities
Repayment of long-term debt									(81,065)	(23,040)	0
Cash dividends									(99,131)	(83,428)	(69,888)
Proceeds from exercise of stock options									0	838	1,094
Taxes paid related to net share settlement of RSUs									(2,311)	(3,550)	(5,128)
Purchase of treasury stock									(36,301)	(42,648)	0
Net cash (used in)/provided by financing activities									940,183	863,278	4,830
Parent Company [Member]
Cash Flows from Operating Activities
Net income									279,135	271,885	176,042
Equity in undistributed earnings of subsidiaries									(44,860)	(154,227)	88,247
(Gain)/loss on equity securities									(4,414)	2,242	0
Write-downs on venture capital and other investments									105	271	254
Loss/(gain) in fair value of warrants									145	(93)	(12)
Stock issued to directors as compensation									749	649	550
Net change in other assets									125	915	3,490
Gain from acquisition									0	(340)	(5,628)
Net change in other liabilities									(832)	(1,375)	5,949
Net cash provided by operating activities									230,153	119,927	268,892
Cash Flows from Investing Activities
(Increase)/decrease in short-term investment									0	(1)	(2)
Proceeds from sale of equity securities									2,829	0	12,580
Purchase of available-for-sale securities									0	0	(2,759)
Venture capital and other investments									399	150	671
Acquisitions, net of cash acquired									0	0	(275,328)
Net cash provided by/(used in) investment activities									3,228	149	(264,838)
Cash Flows from Financing Activities
Repayment of long-term debt									(81,065)	(21,633)	0
Proceeds of issuance of long-term debt									0	0	75,000
Cash dividends									(99,131)	(83,428)	(69,888)
Proceeds from shares issued under the Dividend Reinvestment Plan									3,366	2,821	2,528
Proceeds from exercise of stock options									0	838	1,094
Taxes paid related to net share settlement of RSUs									(2,311)	(3,550)	(5,128)
Purchase of treasury stock									(36,301)	(42,648)	0
Net cash (used in)/provided by financing activities									(215,442)	(147,600)	3,606
(Decrease)/Increase in cash and cash equivalents									17,939	(27,524)	7,660
Cash and cash equivalents, beginning of the year				$ 21,627				$ 49,151	21,627	49,151	41,491
Cash and cash equivalents, end of the year	$ 39,566				$ 21,627				$ 39,566	$ 21,627	$ 49,151