Note 20 - Condensed Financial Information of Cathay General Bancorp (Detail) - Statements of Operations (USD $) In Thousands, 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
Non-interest income/(loss)	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 tax benefit	15,276	17,686	16,619	16,547	14,459	14,162	10,906	11,734	66,128	51,261	(14,629)
Loss before undistributed earnings of subsidiaries	43,735	48,195	46,630	45,611	42,313	40,397	35,358	33,948	184,171	152,016	(2,454)
Net income	28,459	30,509	30,011	29,064	27,854	26,235	24,452	22,214	117,438	100,150	11,565
Parent Company [Member]
Cash dividends from Cathay Bank									154,700
Interest income									196	259	227
Interest expense									3,228	3,038	3,075
Non-interest income/(loss)									3,718	286	(782)
Non-interest expense									2,064	1,548	1,308
Income/(loss) before income tax benefit									153,322	(4,041)	(4,938)
Income tax benefit									(579)	(1,699)	(2,076)
Loss before undistributed earnings of subsidiaries									153,901	(2,342)	(2,862)
Undistributed (loss)/earnings of subsidiaries									(36,463)	102,492	14,427
Net income									$ 117,438	$ 100,150	$ 11,565