LICENSE AGREEMENTS (Details) (USD $)	0 Months Ended	12 Months Ended
Dec. 15, 2014 item	Dec. 31, 2014
LICENSE AGREEMENTS
Amount expected to be recognized as revenue		$ 35,000,000us-gaap_DeferredRevenue
Expected Revenue
LICENSE AGREEMENTS
Amount expected to be recognized as revenue		35,000,000
License agreements \| Janssen Biotech \| Initial Period
LICENSE AGREEMENTS
Upfront payment received		35,000,000
Number of agreed upon studies	2
Percentage of costs to be paid by Geron		50.00%
Percentage of costs to be paid by Janssen		50.00%
Notification period		24 months
License agreements \| Janssen Biotech \| U.S. Opt-In Rights exercised
LICENSE AGREEMENTS
Percentage of costs to be paid by Geron		20.00%
Percentage of costs to be paid by Janssen		80.00%
Continuation Election milestone payment		65,000,000
Aggregate maximum total payments for development and regulatory milestones under collaboration agreement		470,000,000
Aggregate maximum total of payments for sales milestones under collaboration agreement		350,000,000
Tiered royalties percentage, low end of the range		mid-teens
Tiered royalties percentage, high end of the range		low twenties
U.S. selling effort percentage		20.00%
License agreements \| Janssen Biotech \| U.S. Opt-In Rights not exercised
LICENSE AGREEMENTS
Continuation Election milestone payment		65,000,000
Full U.S. Rights fee		70,000,000
Aggregate maximum total payments for development and regulatory milestones under collaboration agreement		415,000,000
Aggregate maximum total of payments for sales milestones under collaboration agreement		$ 350,000,000
Tiered royalties percentage, low end of the range		double-digit
Tiered royalties percentage, high end of the range		mid-teens
License agreements \| Janssen Biotech \| Collaborative Arrangement Development IP Jointly Or Solely Owned
LICENSE AGREEMENTS
Percentage of costs to be paid by Geron		50.00%
Percentage of costs to be paid by Janssen		50.00%

The amount of consideration that Geron could receive upon Janssen providing a positive Continuation Election notice.

No definition available.

Name:	gern_CollaborativeArrangementContinuationElectionMilestonePayment
Namespace Prefix:	gern_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Represents the aggregate maximum consideration that Geron could receive from attaining certain development and regulatory milestone events under the collaboration agreement.

+ References

No definition available.

+ Details

Name:	gern_CollaborativeArrangementDevelopmentAndRegulatoryMilestonesAggregateMaximum
Namespace Prefix:	gern_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Represents the consideration that Geron will receive from Janssen upon Geron's decision to opt-out of further development cost sharing after Janssen's positive Continuation Election notice.

+ References

No definition available.

+ Details

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Namespace Prefix:	gern_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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_CollaborativeArrangementGeronCostSharingPercentage
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_CollaborativeArrangementJanssenCostSharingPercentage
Namespace Prefix:	gern_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the latest notification period from Janssen to Geron in the event of early termination or suspension of the Initial Phase 2 MF Study.

+ References

No definition available.

+ Details

Name:	gern_CollaborativeArrangementNotificationPeriod
Namespace Prefix:	gern_
Data Type:	xbrli:durationItemType
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

Represents the aggregate maximum consideration that Geron could receive from attaining certain sales milestone events under the collaboration agreement.

+ References

No definition available.

+ Details

Name:	gern_CollaborativeArrangementSalesMilestonesAggregateMaximum
Namespace Prefix:	gern_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Represents the percentage of sales effort that Geron can provide in lieu of funding the same percentage of promotion costs upon regulatory approval and commercial launch of imetelstat in the U.S.

+ References

No definition available.

+ Details

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

- Definition

Represents the high end of the range of tiered royalty percentages to be applied to worldwide net sales of imetelstat.

+ References

No definition available.

+ Details

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

- Definition

Represents the low end of the range of tiered royalty percentages to be applied to worldwide net sales of imetelstat.

+ References

No definition available.

+ Details

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

- Definition

Upfront payment received from Janssen upon effectiveness of Collaboration Agreement.

+ References

No definition available.

+ Details

Name:	gern_UpfrontPaymentReceivedFromJanssenUnderCollaborationAgreement
Namespace Prefix:	gern_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

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

- Definition

Amount of deferred revenue as of balance sheet date. Deferred revenue represents collections of cash or other assets related to a revenue producing activity for which revenue has not yet been recognized. Generally, an entity records deferred revenue when it receives consideration from a customer before achieving certain criteria that must be met for revenue to be recognized in conformity with GAAP.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 13

-Section A

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 605

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SAB TOPIC 13.A.4(a).Q1 Response)

-URI http://asc.fasb.org/extlink&oid=27012821&loc=d3e214044-122780

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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Namespace Prefix:
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- Details

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