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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - RESEARCH AND DEVELOPMENT (Details)	3 Months Ended
	Mar. 31, 2020
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Percentage of costs to be paid by Geron after termination of Collaboration Agreement	100.00%

X

- Definition

Represents the percentage of costs that the company must pay after the termination of the Collaboration Agreement.

+ References

No definition available.

+ Details

Name:	gern_CollaborativeArrangementAfterTerminationCostSharingPercentageOfEntity
Namespace Prefix:	gern_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

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- References

No definition available.

+ Details

Name:	us-gaap_OrganizationConsolidationAndPresentationOfFinancialStatementsAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration