Note 19 - Condensed Financial Information of Cathay General Bancorp - Statements of Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash Flows from Operating Activities
Net income	$ 70,893	$ 56,794	$ 54,321	$ 46,852	$ 67,377	$ 72,835	$ 72,244	$ 66,679	$ 228,860	$ 279,135	$ 271,885
Write-downs on venture capital and other investments									58,131	39,898	40,871
Net change in other assets									(21,247)	6,163	(13,984)
Gain from acquisition									0	0	(340)
Net change in other liabilities									(18,948)	21,061	13,848
Net cash provided by operating activities									319,955	434,979	336,538
Cash Flows from Investing Activities
Net cash used in investing activities									(242,204)	(1,381,674)	(1,134,327)
Cash Flows from Financing Activities
Repayment of long-term debt									(7,663)	(81,065)	(23,040)
Cash dividends									(98,688)	(99,131)	(83,428)
Proceeds from exercise of stock options									0	0	838
Taxes paid related to net share settlement of RSUs									(1,911)	(2,311)	(3,550)
Purchase of treasury stock									(23,593)	(36,301)	(42,648)
Net cash provided by financing activities									749,549	940,183	863,278
Parent Company [Member]
Cash Flows from Operating Activities
Net income									228,860	279,135	271,885
Equity in undistributed earnings of subsidiaries									(90,306)	(44,860)	(154,227)
Loss/(gain) on equity securities									641	(4,414)	2,242
Write-downs on venture capital and other investments									107	105	271
Loss/(gain) in fair value of warrants									18	145	(93)
Stock issued to directors as compensation									800	749	649
Net change in other assets									(1,182)	125	915
Gain from acquisition									0	0	(340)
Net change in other liabilities									(9,853)	(832)	(1,375)
Net cash provided by operating activities									129,085	230,153	119,927
Cash Flows from Investing Activities
Increase in short-term investment									0	0	(1)
Proceeds from liquidation of subsidiary									2,399	0	0
Proceeds from sale of equity securities									3,112	2,829	0
Venture capital and other investments									116	399	150
Net cash used in investing activities									5,627	3,228	149
Cash Flows from Financing Activities
Repayment of long-term debt									(7,644)	(81,065)	(21,633)
Cash dividends									(98,688)	(99,131)	(83,428)
Proceeds from shares issued under the Dividend Reinvestment Plan									9,777	3,366	2,821
Proceeds from exercise of stock options									0	0	838
Taxes paid related to net share settlement of RSUs									(1,911)	(2,311)	(3,550)
Purchase of treasury stock									(23,593)	(36,301)	(42,648)
Net cash provided by financing activities									(122,059)	(215,442)	(147,600)
Increase/(decrease) in cash and cash equivalents									12,653	17,939	(27,524)
Cash and cash equivalents, beginning of the year				$ 39,566				$ 21,627	39,566	21,627	49,151
Cash and cash equivalents, end of the year	$ 52,219				$ 39,566				$ 52,219	$ 39,566	$ 21,627