Note 20 - Condensed Financial Information of Cathay General Bancorp - Statements of Cash Flows (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash Flows from Operating Activities
Net income	$ 64,648,000	$ 69,757,000	$ 73,664,000	$ 63,816,000	$ 25,940,000	$ 49,746,000	$ 51,415,000	$ 48,941,000	$ 271,885,000	$ 176,042,000	$ 175,099,000
Adjustments to reconcile net income to net cash provided by operating activities:
Write-downs on venture capital and other investments									40,871,000	4,572,000	28,897,000
Write-down on impaired securities											206,000
Stock issued to directors as compensation									7,974,000	5,734,000	4,963,000
Net change in other assets									(13,984,000)	25,747,000	13,478,000
Gain from acquisition									(340,000)	(5,628,000)
Net change in other liabilities									13,848,000	(2,641,000)	(2,784,000)
Net cash provided by operating activities									336,538,000	248,874,000	234,402,000
Cash Flows from Investing Activities
Proceeds from sale of investment securities available-for-sale									99,899,000	111,704,000	294,000
Purchase of available-for-sale securities									(497,787,000)	(339,814,000)	(941,327,000)
Acquisitions, net of cash acquired										(1,392,000)
Net cash provided by/(used in) investment activities									(1,134,327,000)	(903,987,000)	(853,492,000)
Cash Flows from Financing Activities
Repayment of long-term debt									(23,040,000)
Proceeds from issuance of long-term debt and other borrowings										75,000,000
Cash dividends									(83,428,000)	(69,888,000)	(59,274,000)
Proceeds from exercise of stock options									838,000	1,094,000	7,661,000
Taxes paid related to net share settlement of RSUs									(3,550,000)	(5,128,000)	(103,000)
Purchase of treasury stock									(42,648,000)		(54,441,000)
Net cash (used in)/provided by financing activities									863,278,000	4,830,000	1,087,164,000
(Decrease)/Increase in cash and cash equivalents									65,489,000	(650,283,000)	468,074,000
Cash and cash equivalents, beginning of the year				534,801,000				1,185,084,000	534,801,000	1,185,084,000	717,010,000
Cash and cash equivalents, end of the year	600,290,000				534,801,000				600,290,000	534,801,000	1,185,084,000
Parent Company [Member]
Cash Flows from Operating Activities
Net income									271,885,000	176,042,000	175,099,000
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in undistributed earnings of subsidiaries									(154,227,000)	88,247,000	(67,770,000)
Dividends in excess of earnings of non-bank subsidiaries											1,281,000
Unrealized loss on equity securities									2,242,000
Write-downs on venture capital and other investments									271,000	254,000	503,000
Write-down on impaired securities											206,000
Loss in fair value of warrants									(93,000)	(12,000)	(17,000)
Stock issued to directors as compensation									649,000	550,000	550,000
Net change in other assets									915,000	3,490,000	(1,136,000)
Gain from acquisition									(340,000)	(5,628,000)
Net change in other liabilities									(1,375,000)	5,949,000	(756,000)
Net cash provided by operating activities									119,927,000	268,892,000	107,960,000
Cash Flows from Investing Activities
(Increase)/decrease in short-term investment									(1,000)	(2,000)	23,999,000
Proceeds from sale of investment securities available-for-sale										12,580,000	294,000
Purchase of available-for-sale securities										(2,759,000)
Venture capital and other investments									150,000	671,000	134,000
Acquisitions, net of cash acquired										(275,328,000)
Net cash provided by/(used in) investment activities									149,000	(264,838,000)	24,427,000
Cash Flows from Financing Activities
Repayment of long-term debt									(21,633,000)
Proceeds from issuance of long-term debt and other borrowings										75,000,000
Cash dividends									(83,428,000)	(69,888,000)	(59,274,000)
Proceeds from shares issued under the Dividend Reinvestment Plan									2,821,000	2,528,000	2,277,000
Proceeds from exercise of stock options									838,000	1,094,000	7,661,000
Taxes paid related to net share settlement of RSUs									(3,550,000)	(5,128,000)	(103,000)
Purchase of treasury stock									(42,648,000)		(54,441,000)
Net cash (used in)/provided by financing activities									(147,600,000)	3,606,000	(103,880,000)
(Decrease)/Increase in cash and cash equivalents									(27,524,000)	7,660,000	28,507,000
Cash and cash equivalents, beginning of the year				$ 49,151,000				$ 41,491,000	49,151,000	41,491,000	12,984,000
Cash and cash equivalents, end of the year	$ 21,627,000				$ 49,151,000				$ 21,627,000	$ 49,151,000	$ 41,491,000