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STOCKHOLDERS' EQUITY (Details 10) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
401(k) Plan |  |  |
Matching contribution by employer (as a percent) | 50.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent | 75.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent | 100.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent
Period of vesting of matching contributions |  | 4 years | 4 years
Service period of employees |  | 4 years | 4 years
Unvested amount under the defined-contribution savings plan to be amortized | $ 273,000gern_DefinedContributionPlanUnvestedEmployerMatchingContributionsAmountToBeAmortized |  |
Geron 401K Plan |  |  |
401(k) Plan |  |  |
Operating expenses as a result of accelerated vesting of unvested prior employer matches for employees affected by recent restructurings |  | 266,000gern_DefinedContributionPlanAcceleratedVestingAmountInUnvestedPriorEmployerMatchesForEmployeesAffectedByRestructuring / us-gaap_PlanNameAxis = gern_Geron401KPlanMember | 370,000gern_DefinedContributionPlanAcceleratedVestingAmountInUnvestedPriorEmployerMatchesForEmployeesAffectedByRestructuring / us-gaap_PlanNameAxis = gern_Geron401KPlanMember
Research and development | Geron 401K Plan |  |  |
401(k) Plan |  |  |
Operating expense recorded for vested portion of employer matching contribution | 175,000us-gaap_DefinedContributionPlanCostRecognized / us-gaap_IncomeStatementLocationAxis = us-gaap_ResearchAndDevelopmentExpenseMember / us-gaap_PlanNameAxis = gern_Geron401KPlanMember | 156,000us-gaap_DefinedContributionPlanCostRecognized / us-gaap_IncomeStatementLocationAxis = us-gaap_ResearchAndDevelopmentExpenseMember / us-gaap_PlanNameAxis = gern_Geron401KPlanMember | 616,000us-gaap_DefinedContributionPlanCostRecognized / us-gaap_IncomeStatementLocationAxis = us-gaap_ResearchAndDevelopmentExpenseMember / us-gaap_PlanNameAxis = gern_Geron401KPlanMember
General and administrative | Geron 401K Plan |  |  |
401(k) Plan |  |  |
Operating expense recorded for vested portion of employer matching contribution | $ 143,000us-gaap_DefinedContributionPlanCostRecognized / us-gaap_IncomeStatementLocationAxis = us-gaap_GeneralAndAdministrativeExpenseMember / us-gaap_PlanNameAxis = gern_Geron401KPlanMember | 157,000us-gaap_DefinedContributionPlanCostRecognized / us-gaap_IncomeStatementLocationAxis = us-gaap_GeneralAndAdministrativeExpenseMember / us-gaap_PlanNameAxis = gern_Geron401KPlanMember | 259,000us-gaap_DefinedContributionPlanCostRecognized / us-gaap_IncomeStatementLocationAxis = us-gaap_GeneralAndAdministrativeExpenseMember / us-gaap_PlanNameAxis = gern_Geron401KPlanMember

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- Definition Represents the accelerated vesting amount in unvested prior employer matches for employees affected by restructuring under the defined contribution plan. + References No definition available. + Details
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Namespace Prefix: | gern_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Represents the period of service of an employee must complete to be fully vested in an employer matching contribution. + References No definition available. + Details
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Namespace Prefix: | gern_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Represents the unvested amount of employer matching contributions to be amortized under the defined-contribution savings plan as the corresponding years of service are completed by the employees. + References No definition available. + Details
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Namespace Prefix: | gern_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Represents the period over which matching contributions invested in common stock will vest ratably for each year of service completed by the employee. + References No definition available. + Details
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Namespace Prefix: | gern_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: | us-gaap_
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- Definition The amount of the cost recognized during the period for defined contribution plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6414718&loc=d3e28014-114942 + Details
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Balance Type: | debit
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- Definition Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
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- Details
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Balance Type: |
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- Details
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |