Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2021	Nov. 04, 2021
Cover [Abstract]
Entity Registrant Name	GERON CORP
Entity Central Index Key	0000886744
Document Type	10-Q
Document Period End Date	Sep. 30, 2021
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Common Stock, Shares Outstanding		321,949,041
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
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 BOULEVARD
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 Quarterly Report	true
Document Transition Report	false