| Document and Entity Information | May 27, 2020 |
| Cover [Abstract] | |
| Entity Registrant Name | ARENA PHARMACEUTICALS INC |
| Amendment Flag | false |
| Entity Central Index Key | 0001080709 |
| Document Type | 8-K |
| Document Period End Date | May 27, 2020 |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 000-31161 |
| Entity Tax Identification Number | 23-2908305 |
| Entity Address, Address Line One | 6154 Nancy Ridge Drive |
| Entity Address, City or Town | San Diego |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 92121 |
| 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 |
| Security 12b Title | Common Stock, par value $0.0001 per share |
| Trading Symbol | ARNA |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
