| Cover | Aug. 05, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Aug. 5, 2021 |
| Entity Registrant Name | Arena Pharmaceuticals, Inc. |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 000-31161 |
| Entity Tax Identification Number | 23-2908305 |
| Entity Address, Address Line One | 136 Heber Avenue |
| Entity Address, Address Line Two | Suite 204 |
| Entity Address, City or Town | Park City |
| Entity Address, State or Province | UT |
| Entity Address, Postal Zip Code | 84060 |
| City Area Code | 858 |
| Local Phone Number | 453-7200 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, par value $0.0001 per share |
| Trading Symbol | ARNA |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001080709 |
| Amendment Flag | false |
