| Document And Entity Information | May 26, 2021 |
| Cover [Abstract] | |
| Document, Type | 8-K |
| Amendment Flag | false |
| Document, Period End Date | May 26, 2021 |
| Entity, Registrant Name | CATHAY GENERAL BANCORP |
| Entity Central Index Key | 0000861842 |
| Entity, Emerging Growth Company | 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common stock |
| Trading Symbol | CATY |
| Security Exchange Name | NASDAQ |
