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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash Flows from Operating Activities
Net income	$ 25,940	$ 49,746	$ 51,415	$ 48,941	$ 48,015	$ 46,090	$ 34,831	$ 46,163	$ 176,042	$ 175,099	$ 161,109
Adjustments to reconcile net income to net cash provided by operating activities:
Write-downs on venture capital and other investments									4,572	28,897	25,058
Write-down on impaired securities										206	3,875
Stock issued to directors as compensation									5,734	4,963	4,950
Net change in other assets									25,747	13,478	(3,429)
Net change in other liabilities									(2,641)	(2,784)	(15,506)
Net cash provided by operating activities									248,874	234,402	176,697
Cash Flows from Investing Activities
Proceeds from sale of investment securities available-for-sale									111,704	294	385,234
Purchase of available-for-sale securities									(339,814)	(941,327)	(295,497)
Acquisitions, net of cash acquired									(118,392)		6,572
Net cash (used in)/provided by investment activities									(224,665)	(1,283,679)	(1,177,571)
Cash Flows from Financing Activities
Proceeds from issuance of long-term debt and other borrowings									75,000
Cash dividends									(69,888)	(59,274)	(45,283)
Proceeds from exercise of stock options									1,094	7,661	5,014
Taxes paid related to net share settlement of RSUs									(5,128)	(103)	(227)
Excess tax short-fall from share-based payment arrangements											(5,348)
Purchase of treasury stock										(54,441)	(59,412)
Net cash used in financing activities									4,830	1,087,164	1,004,174
Increase/(Decrease) in cash and cash equivalents									29,039	37,887	3,300
Cash and cash equivalents, beginning of the year				218,017				180,130	218,017	180,130	176,830
Cash and cash equivalents, end of the year	247,056				218,017				247,056	218,017	180,130
Parent Company [Member]
Cash Flows from Operating Activities
Net income									176,042	175,099	161,109
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in undistributed earnings of subsidiaries									88,247	(67,770)	(4,884)
Dividends in excess of earnings of non-bank subsidiaries										1,281
Write-downs on venture capital and other investments									254	503	468
Write-down on impaired securities										206
Loss in fair value of warrants									(12)	(17)
Stock issued to directors as compensation									550	550	495
Excess tax short-fall from stock options											5,348
Net change in other assets									(2,138)	(1,136)	619
Net change in other liabilities									5,949	(756)	(5,438)
Net cash provided by operating activities									268,892	107,960	157,717
Cash Flows from Investing Activities
(Increase)/decrease in short-term investment									(2)	23,999	(1,121)
Proceeds from sale of investment securities available-for-sale									12,580	294
Purchase of available-for-sale securities									(2,759)		(410)
Venture capital and other investments									671	134
Acquisitions, net of cash acquired									(275,328)		(57,006)
Net cash (used in)/provided by investment activities									(264,838)	24,427	(58,537)
Cash Flows from Financing Activities
Proceeds from issuance of long-term debt and other borrowings									75,000
Cash dividends									(69,888)	(59,274)	(45,283)
Proceeds from shares issued under the Dividend Reinvestment Plan									2,528	2,277	4,175
Proceeds from exercise of stock options									1,094	7,661	5,014
Taxes paid related to net share settlement of RSUs									(5,128)	(103)	(227)
Excess tax short-fall from share-based payment arrangements											(5,348)
Purchase of treasury stock										(54,441)	(59,412)
Net cash used in financing activities									3,606	(103,880)	(101,081)
Increase/(Decrease) in cash and cash equivalents									7,660	28,507	(1,901)
Cash and cash equivalents, beginning of the year				$ 41,491				$ 12,984	41,491	12,984	14,885
Cash and cash equivalents, end of the year	$ 49,151				$ 41,491				$ 49,151	$ 41,491	$ 12,984