| Document and Entity Information | Jul. 29, 2020 |
| Cover [Abstract] | |
| Entity Registrant Name | SPECTRUM PHARMACEUTICALS INC |
| Amendment Flag | false |
| Entity Central Index Key | 0000831547 |
| Document Type | 8-K |
| Document Period End Date | Jul. 29, 2020 |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 001-35006 |
| Entity Tax Identification Number | 93-0979187 |
| Entity Address, Address Line One | 11500 S. Eastern Ave. |
| Entity Address, Address Line Two | Ste. 240 |
| Entity Address, City or Town | Henderson |
| Entity Address, State or Province | NV |
| Entity Address, Postal Zip Code | 89052 |
| City Area Code | (702) |
| Local Phone Number | 835-6300 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, $0.001 par value |
| Trading Symbol | SPPI |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
