Note 19 - Condensed Financial Information of Cathay General Bancorp - Statements of Cash Flows (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash Flows from Operating Activities
Net income	$ 48,015,000	$ 46,090,000	$ 34,831,000	$ 46,163,000	$ 41,446,000	$ 38,483,000	$ 45,200,000	$ 35,980,000	$ 175,099,000	$ 161,109,000	$ 137,830,000
Adjustments to reconcile net income to net cash provided by operating activities:
Income associated with debt redemption											(555,000)
Write-downs on venture capital and other investments									28,897,000	25,058,000	436,000
Write-down on impaired securities									206,000	3,875,000	820,000
Stock issued to directors as compensation									4,963,000	4,950,000	4,190,000
Net change in other assets									13,478,000	(3,429,000)	(2,776,000)
Net change in other liabilities									(2,784,000)	(15,506,000)	(11,256,000)
Net cash provided by operating activities									234,402,000	176,697,000	150,071,000
Cash Flows from Investing Activities
Proceeds from sale of investment securities available-for-sale									294,000	385,234,000	160,451,000
Purchase of available-for-sale securities									(941,327,000)	(295,497,000)	(885,782,000)
Acquisition, net of cash acquired										6,572,000
Net cash (used in)/provided by investment activities									(1,283,679,000)	(1,177,571,000)	(459,361,000)
Cash Flows from Financing Activities
Cash dividends									(59,274,000)	(45,283,000)	(23,104,000)
Proceeds from exercise of stock options									7,661,000	5,014,000	128,000
Taxes paid related to net share settlement of RSUs									(103,000)	(227,000)	(850,000)
Purchase of treasury stock									(54,441,000)	(59,412,000)
Net cash used in financing activities									1,087,164,000	1,004,174,000	332,373,000
Increase/(Decrease) in cash and cash equivalents									37,887,000	3,300,000	23,083,000
Cash and cash equivalents, beginning of the year				180,130,000				176,830,000	180,130,000	176,830,000	153,747,000
Cash and cash equivalents, end of the year	218,017,000				180,130,000				218,017,000	180,130,000	176,830,000
Parent Company [Member]
Cash Flows from Operating Activities
Net income									175,099,000	161,109,000	137,830,000
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in undistributed earnings of subsidiaries									(66,489,000)	(4,884,000)	(105,793,000)
Dividends in excess of earnings of non-bank subsidiaries									1,281,000
Gains on sale of securities											(10,689,000)
Income associated with debt redemption											(555,000)
Write-downs on venture capital and other investments									503,000	468,000	432,000
Write-down on impaired securities									206,000		264,000
Loss in fair value of warrants									(17,000)		3,000
Stock issued to directors as compensation									550,000	495,000	350,000
Excess tax short-fall from stock options										5,348,000	1,285,000
Net change in other assets									(1,136,000)	619,000	(3,445,000)
Net change in other liabilities									(756,000)	(5,438,000)	(1,294,000)
Net cash provided by operating activities									107,960,000	157,717,000	18,388,000
Cash Flows from Investing Activities
(Increase)/decrease in short-term investment									23,999,000	(1,121,000)	14,797,000
Proceeds from sale of investment securities available-for-sale									294,000		12,083,000
Purchase of available-for-sale securities										(410,000)	(7,920,000)
Venture capital and other investments									134,000		(590,000)
Acquisition, net of cash acquired										(57,006,000)
Net cash (used in)/provided by investment activities									24,427,000	(58,537,000)	18,370,000
Cash Flows from Financing Activities
Repayment of long-term debt											(1,445,000)
Cash dividends									(59,274,000)	(45,283,000)	(23,104,000)
Proceeds from shares issued under the Dividend Reinvestment Plan									2,277,000	4,175,000	2,848,000
Proceeds from exercise of stock options									7,661,000	5,014,000	128,000
Taxes paid related to net share settlement of RSUs									(103,000)	(227,000)	(850,000)
Excess tax short-fall from share-based payment arrangements										(5,348,000)	(1,285,000)
Purchase of treasury stock									(54,441,000)	(59,412,000)
Net cash used in financing activities									(103,880,000)	(101,081,000)	(23,708,000)
Increase/(Decrease) in cash and cash equivalents									28,507,000	(1,901,000)	13,050,000
Cash and cash equivalents, beginning of the year				$ 12,984,000				$ 14,885,000	12,984,000	14,885,000	1,835,000
Cash and cash equivalents, end of the year	$ 41,491,000				$ 12,984,000				$ 41,491,000	$ 12,984,000	$ 14,885,000