DOCUMENT AND ENTITY INFORMATION - USD ($)	12 Months Ended
	Dec. 31, 2021	Mar. 01, 2022	Jun. 30, 2021
Cover [Abstract]
Entity Registrant Name	GERON CORP
Entity Central Index Key	0000886744
Document Type	10-K
Document Period End Date	Dec. 31, 2021
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	false
Entity Public Float			$ 376,700,000
Entity Common Stock, Shares Outstanding		323,731,591
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	FY
Trading Symbol	GERN
Entity File Number	000-20859
Entity Tax Identification Number	75-2287752
Entity Shell Company	false
Entity Address, Address Line One	919 East Hillsdale Blvd.
Entity Address, Address Line Two	Suite 250
Entity Address, City or Town	Foster City
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94404
City Area Code	650
Local Phone Number	473-7700
Entity Interactive Data Current	Yes
Title of 12(b) Security	Common Stock, $0.001 par value
Security Exchange Name	NASDAQ
Entity Incorporation, State or Country Code	DE
Document Annual Report	true
Document Transition Report	false
Auditor Firm ID	42
Auditor Name	Ernst & Young LLP
Auditor Location	Redwood City, California
Documents Incorporated by Reference	Portions of the Registrant’s definitive proxy statement for the 2022 annual meeting of stockholders to be filed pursuant to Regulation 14A within 120 days of the Registrant’s fiscal year ended December 31, 2021