Note 24 - Quarterly Results of Operations (Unaudited) (Details) - Selected Unaudited Quarterly Financial Data (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Selected Unaudited Quarterly Financial Data [Abstract]
Interest income	$ 106,043	$ 106,335	$ 105,062	$ 101,207	$ 101,621	$ 102,462	$ 100,862	$ 102,051
Interest expense	18,292	19,580	19,445	18,549	19,659	19,854	20,868	21,919	75,866	82,300	108,491
Net interest income	87,751	86,755	85,617	82,658	81,962	82,608	79,994	80,132	342,781	324,696	321,253
Reversal for credit losses	(2,000)	(5,100)	(3,700)			(3,000)			(10,800)	(3,000)	(9,000)
Net-interest income after provision for loan losses	89,751	91,855	89,317	82,658	81,962	85,608	79,994	80,132	353,581	327,696	330,253
Non-interest income	7,973	8,974	9,021	14,559	8,345	16,720	20,361	14,881	40,527	60,307	46,507
Non-interest expense	41,125	42,607	42,513	48,068	40,319	50,670	53,716	49,128	174,313	193,833	192,589
Income before income tax expense	56,599	58,222	55,825	49,149	49,988	51,658	46,639	45,885	219,795	194,170	184,171
Income tax expense	21,021	22,313	20,741	17,890	17,946	19,029	16,573	16,887	81,965	70,435	66,128
Net income	35,578	35,909	35,084	31,259	32,042	32,629	30,066	28,998	137,830	123,143	117,438
Less: net income attributable to noncontrolling interest					140	151	150	151		592	605
Net income attributable to Cathay General Bancorp	35,578	35,909	35,084	31,259	31,902	32,478	29,916	28,847	137,830	123,735	118,043
Dividends on preferred stock						(2,434)	(2,067)	(5,184)		9,685	16,488
Net income available to common stockholders	$ 35,578	$ 35,909	$ 35,084	$ 31,259	$ 31,902	$ 30,044	$ 27,849	$ 23,663	$ 137,830	$ 113,458	$ 100,950
Basic net income attributable to common stockholders per common share (in Dollars per share)	$ 0.45	$ 0.45	$ 0.44	$ 0.39	$ 0.40	$ 0.38	$ 0.35	$ 0.30	$ 1.73	$ 1.44	$ 1.28
Diluted net income attributable to common stockholders per common share (in Dollars per share)	$ 0.44	$ 0.45	$ 0.44	$ 0.39	$ 0.40	$ 0.38	$ 0.35	$ 0.30	$ 1.72	$ 1.43	$ 1.28