| Cover Page - USD ($) $ in Millions | 12 Months Ended | | |
| Dec. 31, 2022 | Apr. 28, 2023 | Jun. 30, 2022 |
| Cover [Abstract] | | | |
| Document Type | 10-K/A | | |
| Document Annual Report | true | | |
| Document Period End Date | Dec. 31, 2022 | | |
| Current Fiscal Year End Date | --12-31 | | |
| Document Transition Report | false | | |
| Entity File Number | 001-35006 | | |
| Entity Registrant Name | SPECTRUM PHARMACEUTICALS, INC. | | |
| Entity Incorporation, State or Country Code | DE | | |
| Entity Tax Identification Number | 93-0979187 | | |
| Entity Address, Address Line One | Pilot House - Lewis Wharf, 2 Atlantic Ave | | |
| Entity Address, Address Line Two | 6th Floor | | |
| Entity Address, City or Town | Boston | | |
| Entity Address, State or Province | MA | | |
| Entity Address, Postal Zip Code | 02110 | | |
| City Area Code | 617 | | |
| Local Phone Number | 586-3900 | | |
| Title of 12(b) Security | Common Stock, $0.001 par value | | |
| Trading Symbol | SPPI | | |
| Security Exchange Name | NASDAQ | | |
| Entity Well-known Seasoned Issuer | No | | |
| Entity Voluntary Filers | No | | |
| Entity Current Reporting Status | Yes | | |
| Entity Interactive Data Current | Yes | | |
| Entity Filer Category | Non-accelerated Filer | | |
| Entity Small Business | true | | |
| Entity Emerging Growth Company | false | | |
| ICFR Auditor Attestation Flag | false | | |
| Entity Shell Company | false | | |
| Entity Public Float | | | $ 115.4 |
| Entity Common Stock, Shares Outstanding | | 205,284,506 | |
| Amendment Flag | true | | |
| Document Fiscal Year Focus | 2022 | | |
| Document Fiscal Period Focus | FY | | |
| Entity Central Index Key | 0000831547 | | |
| Amendment Description | Spectrum Pharmaceuticals, Inc., (“we”, “us”, or the “Company”), is filing this Amendment No. 1 on Form 10-K/A, or Amendment No. 1, to its Annual Report on Form 10-K for the fiscal year ended December 31, 2022, which was filed with the Securities and Exchange Commission on March 31, 2023, or the Original Form 10-K, for the sole purpose of including the information required by Part III of Form 10-K. This information was previously omitted from the Original 10-K in reliance on General Instruction G(3) to Form 10-K, which permits the information in Part III to be incorporated in the Form 10-K by reference from the Company’s definitive proxy statement if such statement is filed no later than 120 days after the Company’s fiscal year-end. The Company is filing this Amendment No. 1 to include Part III information in its Form 10-K because it will not file a definitive proxy statement containing this information within 120 days after the end of the fiscal year covered by the Original Form 10-K. This Amendment No. 1 amends and restates in their entirety Items 10, 11, 12, 13 and 14 of Part III of the Original Form 10-K. Pursuant to Rule 12b-15 under the Securities Exchange Act of 1934, as amended, or the Exchange Act, this Amendment No. 1 also contains certifications pursuant to Section 302 of the Sarbanes-Oxley Act of 2002, which are attached hereto. Because no financial statements have been included in this Amendment No. 1 and this Amendment No. 1 does not contain or amend any disclosure with respect to Items 307 and 308 of Regulation S-K, paragraphs 3, 4, and 5 of the certifications have been omitted. Except as explicitly set forth herein, this Amendment No. 1 does not purport to modify or update the disclosures in, or exhibits to, the Original Form 10-K or to update the Original Form 10-K to reflect events occurring after the date of such filing. | | |
| Auditor Name | RSM US LLP | | |
| Auditor Firm ID | 49 | | |
| Auditor Location | Los Angeles, California | | |
