Cover Page - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2021	Feb. 17, 2022	Jun. 30, 2021
Cover [Abstract]
Entity Registrant Name	ARENA PHARMACEUTICALS, INC.
Entity Central Index Key	0001080709
Document Type	10-K/A
Document Period End Date	Dec. 31, 2021
Current Fiscal Year End Date	--12-31
Entity File Number	000-31161
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	FY
Amendment Flag	false
Trading Symbol	ARNA
Entity Common Stock, Shares Outstanding		61,659,385
Entity Voluntary Filers	No
Document Annual Report	true
Document Transition Report	false
Entity Address, Postal Zip Code	84060
Entity Tax Identification Number	23-2908305
Local Phone Number	453.7200
City Area Code	858
Entity Incorporation, State or Country Code	DE
Entity Address, Address Line One	136 Heber Avenue
Entity Address, Address Line Two	Suite 204
Entity Address, City or Town	Park City
Entity Address, State or Province	UT
Title of 12(b) Security	Common Stock, par value $0.0001 per share
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Shell Company	false
Entity Small Business	false
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Documents Incorporated by Reference	None.
Entity Filer Category	Large Accelerated Filer
Entity Public Float			$ 4.2