Quarterly Results Of Operations (Selected Unaudited Quarterly Financial Data) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Quarterly Results Of Operations [Abstract]
Interest income	$ 111,076	$ 114,379	$ 114,339	$ 113,777	$ 118,332	$ 120,506	$ 124,287	$ 126,469	$ 453,571	$ 489,594	$ 528,731
Interest expense	31,759	33,426	36,024	38,672	43,095	47,165	49,680	51,748	139,881	191,688	246,039
Net interest income before provision for credit losses	79,317	80,953	78,315	75,105	75,237	73,341	74,607	74,721	313,690	297,906	282,692
Provision for credit losses	2,000	9,000	10,000	6,000	10,000	17,900	45,000	84,000	27,000	156,900	307,000
Net interest income/(loss) after provision for credit losses	77,317	71,953	68,315	69,105	65,237	55,441	29,607	(9,279)	286,690	141,006	(24,308)
Non-interest income	8,986	16,827	12,453	12,626	16,169	3,886	7,412	4,784	50,892	32,251	78,654
Non-interest expense	43,990	48,383	45,410	47,783	56,348	34,881	40,319	44,163	185,566	175,711	183,037
Income/(loss) before income tax (benefit)/expense	42,313	40,397	35,358	33,948	25,058	24,446	(3,300)	(48,658)	152,016	(2,454)	(128,691)
Income tax expense/(benefit)	14,459	14,162	10,906	11,734	6,789	7,023	(5,373)	(23,068)	51,261	(14,629)	(61,912)
Net income/(loss)	27,854	26,235	24,452	22,214	18,269	17,423	2,073	(25,590)	100,755	12,175	(66,779)
Less: net income attributable to noncontrolling interest	153	151	150	151	158	151	150	151	605	610	611
Net income/(loss) attributable to Cathay General Bancorp	27,701	26,084	24,302	22,063	18,111	17,272	1,923	(25,741)	100,150	11,565	(67,390)
Dividends on preferred stock	(4,114)	(4,111)	(4,107)	(4,105)	(4,102)	(4,098)	(4,096)	(4,092)	(16,437)	(16,388)	(16,338)
Net income/(loss) attributable to common stockholders	$ 23,587	$ 21,973	$ 20,195	$ 17,958	$ 14,009	$ 13,174	$ (2,173)	$ (29,833)	$ 83,713	$ (4,823)	$ (83,728)
Basic net income/(loss) attributable to common stockholders per common share	$ 0.30	$ 0.28	$ 0.26	$ 0.23	$ 0.18	$ 0.17	$ (0.03)	$ (0.41)	$ 1.06	$ (0.06)	$ (1.59)
Diluted net income/(loss) attributable to common stockholders per common share	$ 0.30	$ 0.28	$ 0.26	$ 0.23	$ 0.18	$ 0.17	$ (0.03)	$ (0.41)	$ 1.06	$ (0.06)	$ (1.59)