Note 20 - Condensed Financial Information of Cathay General Bancorp - Statements of Operations (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Interest income	$ 183,869	$ 176,866	$ 167,520	$ 159,700	$ 155,640	$ 154,078	$ 135,629	$ 130,804
Interest expense	38,428	31,782	27,489	24,357	22,593	20,882	18,277	18,690	$ 122,056	$ 80,442	$ 81,200
Gain from acquisition									340	5,628
Non-interest expense	74,395	65,964	63,088	60,971	66,407	61,248	56,658	51,886	264,419	236,199	224,690
Total income tax expense, amount	17,193	18,698	11,046	18,866	51,166	35,163	15,431	20,505	65,802	122,265	67,101
Income before undistributed earnings of subsidiaries	81,841	88,455	84,710	82,682	77,106	84,909	66,846	69,446
Net income	$ 64,648	$ 69,757	$ 73,664	$ 63,816	$ 25,940	$ 49,746	$ 51,415	$ 48,941	271,885	176,042	175,099
Parent Company [Member]
Interest income									57	221	48
Interest expense									9,813	7,637	5,791
Non-interest (loss)/income									(1,940)	1,909	(488)
Gain from acquisition									340	5,628
Non-interest expense									3,782	6,726	3,756
Income before income tax benefit									112,687	258,602	104,411
Total income tax expense, amount									(4,971)	(5,687)	(4,199)
Income before undistributed earnings of subsidiaries									117,658	264,289	108,610
Undistributed earnings of subsidiary									154,227	(88,247)	66,489
Net income									271,885	176,042	175,099
Parent Company [Member] | Cathay Bank [Member]
Cash dividends from Cathay Bank and Far East National Bank									127,825	265,207	113,448
Parent Company [Member] | GBC Venture Capital [Member]
Cash dividends from Cathay Bank and Far East National Bank											$ 950