FORMER COLLABORATION AGREEMENT (Details)					9 Months Ended
	Jun. 14, 2019 USD ($)	Sep. 28, 2018	Sep. 27, 2018	Dec. 15, 2014 item	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Former Collaborative Arrangement [Line Items]
Percentage of costs to be paid by Geron after termination of Collaboration Agreement					100.00%
Amount due to Janssen Biotech, Inc.					$ 1,208,000	$ 2,610,000
Former Collaboration Agreement
Former Collaborative Arrangement [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.					$ 1,208,000
Clinical Supply Agreement
Former Collaborative Arrangement [Line Items]
Expected payment for drug product upon shipment	$ 7,500,000
Clinical Supply Agreement | Maximum
Former Collaborative Arrangement [Line Items]
Expected payment for drug substance and raw materials upon testing and confirmation	$ 6,700,000