Note 19 - Condensed Financial Information of Cathay General Bancorp - Statements of Operations (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2015
Interest income	$ 155,640	$ 154,078	$ 135,629	$ 130,804	$ 130,668	$ 124,155	$ 121,902	$ 122,345
Interest expense	22,593	20,882	18,277	18,690	20,766	20,331	20,126	19,977	$ 80,442	$ 81,200	$ 73,964
Gain from acquisition									5,628
Non-interest expense	66,407	61,248	56,658	51,886	53,503	50,737	68,879	51,571	236,199	224,690	202,720
Income tax benefit	51,166	35,163	15,431	20,505	16,345	15,808	12,273	22,675	122,265	67,101	59,987	$ 59,987
Income before undistributed earnings of subsidiaries	77,106	84,909	66,846	69,446	64,360	61,898	47,104	68,838	298,307	242,200	221,096
Net income	$ 25,940	$ 49,746	$ 51,415	$ 48,941	$ 48,015	$ 46,090	$ 34,831	$ 46,163	176,042	175,099	161,109
Parent Company [Member]
Interest income									221	48	68
Interest expense									7,637	5,791	5,776
Non-interest income/(loss)									1,909	(488)	(1,858)
Gain from acquisition									5,628
Non-interest expense									6,726	3,756	4,644
Income before income tax benefit									258,602	104,411	151,091
Income tax benefit									(5,687)	(4,199)	(5,134)
Income before undistributed earnings of subsidiaries									264,289	108,610	156,225
Undistributed earnings of subsidiary									(88,247)	66,489	4,884
Net income									176,042	175,099	161,109
Parent Company [Member] | Cathay Bank [Member]
Cash dividends from Cathay Bank and Far East National Bank									265,207	113,448	163,301
Parent Company [Member] | GBC Venture Capital [Member]
Cash dividends from Cathay Bank and Far East National Bank										$ 950