Note 23 - Quarterly Results of Operations (Unaudited) (Detail) - Selected Unaudited Quarterly Financial Data (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Interest income	$ 105,281	$ 106,747	$ 107,581	$ 110,135	$ 111,076	$ 114,379	$ 114,339	$ 113,777
Interest expense	24,216	26,330	28,461	29,484	31,759	33,426	36,024	38,672	108,491	139,881	191,688
Net interest income	81,065	80,417	79,120	80,651	79,317	80,953	78,315	75,105	321,253	313,690	297,906
(Reversal)/provision for credit losses			(5,000)	(4,000)	2,000	9,000	10,000	6,000	(9,000)	27,000	156,900
Net-interest income after provision for loan losses	81,065	80,417	84,120	84,651	77,317	71,953	68,315	69,105	330,253	286,690	141,006
Non-interest income	12,202	15,622	9,852	8,831	8,986	16,827	12,453	12,626	46,507	50,892	32,251
Non-interest expense	49,532	47,844	47,342	47,871	43,990	48,383	45,410	47,783	192,589	185,566	175,711
Income before income tax expense	43,735	48,195	46,630	45,611	42,313	40,397	35,358	33,948	184,171	152,016	(2,454)
Income tax expense	15,276	17,686	16,619	16,547	14,459	14,162	10,906	11,734	66,128	51,261	(14,629)
Net income	28,459	30,509	30,011	29,064	27,854	26,235	24,452	22,214	117,438	100,150	11,565
Less: net income attributable to noncontrolling interest	153	151	150	151	153	151	150	151	605	605	610
Net income attributable to Cathay General Bancorp	28,306	30,358	29,861	28,913	27,701	26,084	24,302	22,063	118,043	100,755	12,175
Dividends on preferred stock	(4,127)	(4,123)	(4,121)	(4,117)	(4,114)	(4,111)	(4,107)	(4,105)	16,488	16,437	16,388
Net income available to common stockholders	$ 24,179	$ 26,235	$ 25,740	$ 24,796	$ 23,587	$ 21,973	$ 20,195	$ 17,958	$ 100,950	$ 83,713	$ (4,823)
Basic net income attributable to common stockholders per common share (in Dollars per share)	$ 0.31	$ 0.33	$ 0.33	$ 0.32	$ 0.30	$ 0.28	$ 0.26	$ 0.23	$ 1.28	$ 1.06	$ (0.06)
Diluted net income attributable to commonstockholders per common share (in Dollars per share)	$ 0.31	$ 0.33	$ 0.33	$ 0.32	$ 0.30	$ 0.28	$ 0.26	$ 0.23	$ 1.28	$ 1.06	$ (0.06)