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

No definition available.

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

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

Represents the percentage of costs that Janssen must pay during certain phases of the agreement.

+ References

No definition available.

+ Details

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

- Definition

Represents the percentage of costs that Geron must pay during certain phases of the agreement.

+ References

No definition available.

+ Details

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

- Definition

Represents the number of agreed upon studies.

+ References

No definition available.

+ Details

Name:	gern_CollaborativeArrangementNumberOfStudies
Namespace Prefix:	gern_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Carrying amount as of the balance sheet date of obligations due to counterparty. For classified balance sheets, represents the current portion of such liabilities (due within one year or within the normal operating cycle if longer).

+ References

No definition available.

+ Details

Name:	gern_DueToCounterParty
Namespace Prefix:	gern_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Former collaborative arrangement.

+ References

No definition available.

+ Details

Name:	gern_FormerCollaborativeArrangementLineItems
Namespace Prefix:	gern_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_TypeOfArrangementAxis=gern_FormerCollaborativeArrangementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=gern_JanssenBiotechIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=gern_ClinicalSupplyAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

FORMER COLLABORATION AGREEMENT (Details)				3 Months Ended
FORMER COLLABORATION AGREEMENT (Details)	Sep. 28, 2018	Sep. 27, 2018	Dec. 15, 2014 item	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
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.				$ 1,000	$ 14,269,000
Former Collaboration Agreement \| Janssen Biotech
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.				1,000
Clinical Supply Agreement \| Janssen Biotech
Former Collaborative Arrangement [Line Items]
Amount due to Janssen Biotech, Inc.				$ 0	$ 14,269,000