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

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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

Collaborative arrangement drug supply procurement during transition period maximum.

+ References

No definition available.

+ Details

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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

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

Represents the least number of products required to be developed under the License Agreement.

+ References

No definition available.

+ Details

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

- Definition

License agreement cost sharing percentage of counterparty.

+ References

No definition available.

+ Details

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

- Definition

License agreement cost sharing percentage of entity.

+ References

No definition available.

+ Details

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

- Definition

License agreement maximum additional potential payment to be received upon achievement of certain development and regulatory milestones.

+ References

No definition available.

+ Details

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Namespace Prefix:	gern_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents the range of tiered royalty percentages to be applied to worldwide net sales of licensed products under the License Agreement.

+ References

No definition available.

+ Details

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

- Definition

Upfront payment under License Agreement.

+ References

No definition available.

+ Details

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Definition

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130543-203045

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130539-203045

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130545-203045

+ Details

Name:	us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

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

- Details

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

- Details

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

- Details

Name:	srt_CounterpartyNameAxis=gern_JanssenPharmaceuticalsIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
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LICENSE AGREEMENTS (Details)					3 Months Ended									12 Months Ended
LICENSE AGREEMENTS (Details)	Sep. 28, 2018	Sep. 27, 2018	Sep. 15, 2016 USD ($) item	Dec. 15, 2014 USD ($) item	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount due to Janssen Biotech, Inc.					$ 2,610,000				$ 1,702,000					$ 2,610,000	$ 1,702,000
License fees and royalties					375,000	$ 165,000	$ 208,000	$ 318,000	$ 191,000	$ 163,000	$ 174,000	$ 537,000		1,066,000	$ 1,065,000	$ 6,162,000
Janssen Biotech \| License agreements
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received				$ 35,000,000
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%
Maximum period to supply imetelstat during transition period	24 months
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.					$ 2,610,000									$ 2,610,000
Janssen Biotech \| 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				50.00%
Janssen Pharmaceuticals Inc \| License agreements
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Least number of products under License Agreement \| item			1
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
License fees and royalties													$ 5,000,000
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%