Note 19 - Condensed Financial Information of Cathay General Bancorp - Statements of Operations (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Interest income	$ 167,596	$ 172,234	$ 174,008	$ 186,736	$ 191,226	$ 197,831	$ 192,449	$ 187,761
Interest expense	27,776	34,730	39,533	46,425	50,015	50,831	49,070	44,445	$ 148,464	$ 194,361	$ 122,056
Gain from acquisition									0	0	340
Non-interest expense	75,046	75,997	67,268	65,154	71,192	65,580	69,546	70,970	283,465	277,288	264,419
Income tax benefit	10,332	2,190	3,492	9,091	16,290	20,973	14,383	18,588	25,105	70,234	65,802
Income before undistributed earnings of subsidiaries	81,225	58,984	57,813	55,943	83,667	93,808	86,627	85,267
Net income	$ 70,893	$ 56,794	$ 54,321	$ 46,852	$ 67,377	$ 72,835	$ 72,244	$ 66,679	228,860	279,135	271,885
Parent Company [Member]
Cash dividends from Cathay Bank									146,000	238,998	127,825
Interest income									49	90	57
Interest expense									5,906	8,415	9,813
Non-interest (loss)/income									(435)	4,634	(1,940)
Gain from acquisition									0	0	340
Non-interest expense									4,846	3,491	3,782
Income before income tax benefit									134,862	231,816	112,687
Income tax benefit									(3,692)	(2,459)	(4,971)
Income before undistributed earnings of subsidiaries									138,554	234,275	117,658
Undistributed earnings of subsidiary									90,306	44,860	154,227
Net income									$ 228,860	$ 279,135	$ 271,885