Document And Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2020	Feb. 15, 2021	Jun. 30, 2020
Document Information [Line Items]
Entity Central Index Key	0000861842
Entity Registrant Name	Cathay General Bancorp
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Document Fiscal Period Focus	FY
Document Fiscal Year Focus	2020
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2020
Document Transition Report	false
Entity File Number	001-31830
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	95-4274680
Entity Address, Address Line One	777 North Broadway
Entity Address, City or Town	Los Angeles
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	90012
City Area Code	213
Local Phone Number	625-4700
Title of 12(b) Security	Common Stock, $0.01 par value
Trading Symbol	CATY
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Public Float			$ 1,987,485,927
Entity Common Stock, Shares Outstanding		79,510,444