| Document and Entity Information | Sep. 30, 2021 |
| Cover [Abstract] | |
| Security Exchange Name | NASDAQ |
| Amendment Flag | false |
| Entity Central Index Key | 0001369568 |
| Document Type | 8-K |
| Document Period End Date | Sep. 30, 2021 |
| Entity Registrant Name | CATALYST PHARMACEUTICALS, INC. |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 001-33057 |
| Entity Tax Identification Number | 76-0837053 |
| Entity Address, Address Line One | 355 Alhambra |
| Entity Address, Address Line Two | Circle |
| Entity Address, Address Line Three | Suite 801 |
| Entity Address, City or Town | Coral Gables |
| Entity Address, State or Province | FL |
| Entity Address, Postal Zip Code | 33134 |
| City Area Code | (305) |
| Local Phone Number | 420-3200 |
| Security 12b Title | Common Stock, par value $0.001 per share |
| Trading Symbol | CPRX |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
