Note 20 - Condensed Financial Information of Cathay General Bancorp - Statements of Operations (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Interest income	$ 191,226	$ 197,831	$ 192,449	$ 187,761	$ 183,869	$ 176,866	$ 167,520	$ 159,700
Interest expense	50,015	50,831	49,070	44,445	38,428	31,782	27,489	24,357	$ 194,361	$ 122,056	$ 80,442
Gain from acquisition.									0	340	5,628
Non-interest expense	71,192	65,580	69,546	70,970	74,395	65,964	63,088	60,971	277,288	264,419	236,199
Total income tax expense, amount	16,290	20,973	14,383	18,588	17,193	18,698	11,046	18,866	70,234	65,802	122,265
Income before undistributed earnings of subsidiaries	83,667	93,808	86,627	85,267	81,841	88,455	84,710	82,682
Net income	$ 67,377	$ 72,835	$ 72,244	$ 66,679	$ 64,648	$ 69,757	$ 73,664	$ 63,816	279,135	271,885	176,042
Parent Company [Member]
Interest income									90	57	221
Interest expense									8,415	9,813	7,637
Non-interest income/(loss)									4,634	(1,940)	1,909
Gain from acquisition.									0	340	5,628
Non-interest expense									3,491	3,782	6,726
Income before income tax benefit									231,816	112,687	258,602
Total income tax expense, amount									(2,459)	(4,971)	(5,687)
Income before undistributed earnings of subsidiaries									234,275	117,658	264,289
Undistributed earnings of subsidiary									44,860	154,227	(88,247)
Net income									279,135	271,885	176,042
Parent Company [Member] | Cathay Bank [Member]
Cash dividends from Cathay Bank and Far East National Bank									$ 238,998	$ 127,825	$ 265,207