LICENSE AGREEMENTS (Details)	Jun. 14, 2019 USD ($)	Sep. 28, 2018	Sep. 27, 2018	Sep. 15, 2016 USD ($)	Dec. 15, 2014 item	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount due to Janssen Biotech, Inc.						$ 14,269,000	$ 2,610,000
Janssen Biotech | Former Collaboration Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of agreed upon studies | item					2
Percentage of costs to be paid by Geron					50.00%
Percentage of costs to be paid by Janssen					50.00%
Percentage of costs to be paid by Geron before termination of Collaboration Agreement			50.00%
Percentage of costs to be paid by Geron after termination of Collaboration Agreement		100.00%
Amount due to Janssen Biotech, Inc.						$ 14,269,000
Janssen Biotech | Clinical Supply Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Expected payment for drug product upon shipment	$ 7,500,000
Janssen Biotech | Clinical Supply Agreement | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Expected payment for drug substance and raw materials upon testing and confirmation	$ 6,700,000
Janssen Pharmaceuticals Inc | License agreements
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received under license agreement				$ 5,000,000
Aggregate maximum total payments for development and regulatory milestones under license agreement				$ 75,000,000
Tiered royalties percentage range				low single digit
Janssen Pharmaceuticals Inc | License Arrangement IP Exclusively Licensed
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of costs to be paid by Geron				50.00%
Percentage of costs to be paid by Janssen Pharmaceuticals				50.00%