Note 20 - Condensed Financial Information of Cathay General Bancorp (Details) - Statements of Cash Flows - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash flows from Operating Activities
Net income	$ 41,446,000	$ 38,483,000	$ 45,200,000	$ 35,980,000	$ 35,578,000	$ 35,909,000	$ 35,084,000	$ 31,259,000	$ 161,109,000	$ 137,830,000	$ 123,143,000
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									25,058,000	436,000	409,000
Write-downs on impaired securities									3,875,000	820,000
Stock issued to directors as compensation									4,950,000	4,190,000	3,213,000
Net change in other assets									(3,429,000)	(2,776,000)	23,291,000
Net change in other liabilities									(15,506,000)	(11,256,000)	(4,973,000)
Net cash provided by operating activities									176,697,000	150,071,000	115,073,000
Cash flows from Investment Activities
Proceeds from sale of available-for-sale securities									385,234,000	160,451,000	575,358,000
Purchase of available-for-sale securities									(295,497,000)	(885,782,000)	(350,130,000)
Acquisition, net of cash acquired									6,572,000
Net cash (used in)/provided by investment activities									(1,177,571,000)	(459,361,000)	(311,806,000)
Cash flows from Financing Activities
Redemption of Series B preferred stock											(258,000,000)
Cash dividends									(45,283,000)	(23,104,000)	(12,606,000)
Proceeds from exercise of stock options									5,014,000	128,000	14,755,000
Taxes paid related to net share settlement of RSUs									(227,000)	(850,000)
Purchase of treasury stock									59,412,000
Net cash used in financing activities									1,004,174,000	332,373,000	205,571,000
(Decrease)/increase in cash and cash equivalents									3,300,000	23,083,000	8,838,000
Cash and cash equivalents, beginning of the year				176,830,000				153,747,000	176,830,000	153,747,000	144,909,000
Cash and cash equivalents, end of the year	180,130,000				176,830,000				180,130,000	176,830,000	153,747,000
Parent Company [Member]
Cash flows from Operating Activities
Net income									161,109,000	137,830,000	123,143,000
Adjustments to reconcile net income to net cash provided by operating activities:
Dividends in excess of earnings of subsidiaries											12,078,000
Equity in undistributed earnings of subsidiaries									(4,884,000)	(105,793,000)
Gains on sale of securities										(10,689,000)
Income associated with debt redemption										(555,000)
Write-downs on venture capital and other investments									468,000	432,000	357,000
Write-downs on impaired securities										264,000
Loss in fair value of warrants										3,000	56,000
Stock issued to directors as compensation									495,000	350,000
Excess tax short-fall from stock options									5,348,000	1,285,000	2,509,000
Net change in other assets									619,000	(3,445,000)	(1,684,000)
Net change in other liabilities									(5,438,000)	(1,294,000)	27,000
Net cash provided by operating activities									157,717,000	18,388,000	136,486,000
Cash flows from Investment Activities
(Increase)/decrease in short-term investment									(1,121,000)	14,797,000	123,300,000
Proceeds from sale of available-for-sale securities										12,083,000
Purchase of available-for-sale securities									(410,000)	(7,920,000)
Venture capital and other investments										(590,000)	(835,000)
Acquisition, net of cash acquired									(57,006,000)
Net cash (used in)/provided by investment activities									(58,537,000)	18,370,000	122,465,000
Cash flows from Financing Activities
Repayment of long-term debt										(1,445,000)
Cash dividends									(45,283,000)	(23,104,000)	(12,606,000)
Proceeds from shares issued under the Dividend Reinvestment Plan									4,175,000	2,848,000	605,000
Proceeds from exercise of stock options									5,014,000	128,000	14,755,000
Taxes paid related to net share settlement of RSUs									(227,000)	(850,000)
Excess tax short-fall from share-based payment arrangements									(5,348,000)	(1,285,000)	(2,509,000)
Purchase of treasury stock									(59,412,000)
Net cash used in financing activities									(101,081,000)	(23,708,000)	(257,755,000)
(Decrease)/increase in cash and cash equivalents									(1,901,000)	13,050,000	1,196,000
Cash and cash equivalents, beginning of the year				$ 7,420,000				$ 1,835,000	7,420,000	1,835,000	639,000
Cash and cash equivalents, end of the year	$ 5,048,000				$ 7,420,000				$ 5,048,000	$ 7,420,000	1,835,000
Parent Company [Member] | Series B Preferred Stock [Member]
Cash flows from Financing Activities
Redemption of Series B preferred stock											$ (258,000,000)