| Cover Page | Jun. 17, 2021 | May 28, 2021 |
| Cover [Abstract] | ||
| Document Type | 8-K | |
| Document Period End Date | Jun. 17, 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 |
