Accrued end of term charge.

No definition available.

Name:	gern_AccruedEndOfTermCharge
Namespace Prefix:	gern_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

First date to available to drawdown any amount under a tranche.

+ References

No definition available.

+ Details

Name:	gern_BorrowingAvailableStartDate
Namespace Prefix:	gern_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Minimum cash covenant required upon certain licensing transactions being executed.

+ References

No definition available.

+ Details

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

- Definition

Minimum cash covenant required upon achievement of certain regulatory milestones.

+ References

No definition available.

+ Details

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

- Definition

Debt Covenant Minimum Cash Balance

+ References

No definition available.

+ Details

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

- Definition

Percentage of outstanding loan amount payable upon maturity.

+ References

No definition available.

+ Details

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

- Definition

Minimum amount allowed to be prepaid under debt instrument.

+ References

No definition available.

+ Details

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

- Definition

Last date to drawdown any amounts under a tranche.

+ References

No definition available.

+ Details

Name:	gern_ExpiryDateOfTranche
Namespace Prefix:	gern_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Added percentage to current interest rate under debt instrument to past due amounts.

+ References

No definition available.

+ Details

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

- Definition

Percentage of prepayment amount to be paid upon prepayment under debt instrument.

+ References

No definition available.

+ Details

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

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=120520924&loc=SL6031897-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

+ Details

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

- Definition

Description of any adjustments made to the stated rate to determine the effective rate.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28551-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

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

- Definition

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(2))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

Name:	us-gaap_DebtInstrumentMaturityDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Description of the payment terms of the debt instrument (for example, whether periodic payments include principal and frequency of payments) and discussion about any contingencies associated with the payment.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(3))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

+ Details

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

- Definition

Amount of unused borrowing capacity under the long-term financing arrangement that is available to the entity as of the balance sheet date.

+ References

+ Details

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

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

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

- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=120520924&loc=SL6031897-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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

- Definition

Description of long-term debt arrangements, which are debt arrangements that originally require full repayment more than twelve months after issuance or greater than the normal operating cycle of the company, if longer, and disclosures pertaining to the underlying arrangements, including repayment terms, conversion features, interest rates, restrictions on assets and activities, debt covenants, and other matters important to users of the financial statements. Types of long-term debt arrangements include borrowing under notes payable, bonds payable, debentures, term loans, and other contractual obligations for payment.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21475-112644

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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

- Definition

The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

+ References

+ Details

Name:	us-gaap_UnamortizedDebtIssuanceExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_DebtInstrumentAxis=gern_HerculesAndSiliconValleyBankMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementScenarioAxis=srt_ScenarioForecastMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=gern_TrancheAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=gern_TrancheBMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=gern_TrancheCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

DEBT - Additional Information (Details) - Hercules and Silicon Valley Bank [Member] - USD ($)		9 Months Ended	12 Months Ended
	Jun. 01, 2022	Sep. 30, 2020	Dec. 31, 2020
Debt Instrument [Line Items]
Maximum borrowing capacity under term loan		$ 75,000,000
Description of maturity date terms for term loan			The Term Loan matures on October 1, 2024, or the Loan Maturity Date, and may be extended up to an additional 12 months upon the achievement of certain clinical, regulatory and financial milestones.
Term loan interest rate description			The Term Loan bears interest at a floating rate per annum equal to the greater of either (i) 9.0% or (ii) 9.0% plus the prime rate as reported in The Wall Street Journal (3.25% as of December 31, 2020) less 3.25%.
Term loan maturity date		Oct. 01, 2024
Percentage added to prime rate for debt instrument interest rate		9.00%
End of term charge for loan, percentage		6.55%
Minimum amount of prepayment allowed under debt instrument		$ 5,000,000
Prepayment charge (as a percentage)		1.50%
Description of term loan payment terms		At our option, upon at least five business days’ prior written notice to Hercules, we may prepay all or any portion greater than or equal to $5,000,000 of the outstanding loan by paying the entire principal balance (or portion thereof) and all accrued and unpaid interest. Such prepayment is subject to a prepayment charge of 1.5% of the prepayment amount, if the prepayment is made in any of the first 36 months following the Closing Date. Thereafter, any prepayment is not subject to a prepayment charge.
Debt covenant minimum cash balance upon licensing transaction		$ 30,000,000
Debt covenant minimum cash balance upon regulatory milestone achievement		$ 20,000,000
Additional percentage of interest on past due amounts		5.00%
Scenario Forecast [Member]
Debt Instrument [Line Items]
Debt covenant minimum cash balance	$ 25,000,000
Tranche A [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity under term loan		$ 35,000,000
Principal amount outstanding under term loan		25,000,000	$ 25,000,000
Unused amount under Tranche A term loan		$ 10,000,000
Expiration date to borrow under a tranche for a debt instrument		Jun. 15, 2021
Minimum interest rate (as a percentage)			9.00%
Carrying value of term loan, net			$ 24,042,000
Unamortized debt discount and issuance costs			1,063,000
Accrued end of term charge			105,000
Tranche B [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity under term loan		$ 15,000,000
Principal amount outstanding under term loan			0
Expiration date to borrow under a tranche for a debt instrument		Dec. 15, 2021
Start date to borrow under a tranche for a debt instrument		Jan. 01, 2021
Tranche C [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity under term loan		$ 25,000,000
Principal amount outstanding under term loan			$ 0
Expiration date to borrow under a tranche for a debt instrument		Dec. 31, 2022