| Cover Page | May 26, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | May 26, 2021 |
| Entity Registrant Name | SPECTRUM PHARMACEUTICALS INC |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-35006 |
| Entity Tax Identification Number | 93-0979187 |
| Entity Address, Address Line One | 11500 South Eastern Avenue |
| Entity Address, Address Line Two | Suite 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 |
| Title of 12(b) Security | Common Stock, $0.001 par value |
| Trading Symbol | SPPI |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
| Amendment Flag | false |
| Entity Central Index Key | 0000831547 |
