Cover Page - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Mar. 10, 2022	Jun. 30, 2021
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2021
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	11500 South Eastern Avenue
Entity Address, Address Line Two	Suite 220
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
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			$ 353.9
Entity Common Stock, Shares Outstanding		177,151,513
Documents Incorporated by Reference	Certain information required by Parts II and III are omitted from this Annual Report on Form 10 K and incorporated by reference to our definitive proxy statement for our 2022 annual meeting of shareholders (“2022 Proxy Statement”), to be filed pursuant to Regulation 14A of the Securities Exchange Act of 1934, as amended, or the Exchange Act. If our 2022 Proxy Statement is not filed within 120 days after the end of the fiscal year covered by this Annual Report on Form 10-K, the omitted information will be included in an amendment to this Annual Report on Form 10-K filed not later than the end of such 120-day period.
Amendment Flag	false
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	FY
Entity Central Index Key	0000831547