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STOCKHOLDERS' EQUITY - 401(k) PLAN (Details) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2015 | Dec. 31, 2013
401(k) Plan |  |
Period of vesting of matching contributions |  | 4 years
Service period of employees |  | 4 years
Operating expenses as a result of accelerated vesting of unvested prior employer matches for employees affected by recent restructurings | $ 53,000 | $ 266,000
Unvested amount under the defined-contribution savings plan to be amortized | $ 111,000 |

X
- 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
Name: | gern_DefinedContributionPlanAcceleratedVestingAmountInUnvestedPriorEmployerMatchesForEmployeesAffectedByRestructuring
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
Name: | gern_DefinedContributionPlanServicePeriodOfEmployeesToBeFullyVestedInEmployerMatchingContribution
Namespace Prefix: | gern_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- 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
Name: | gern_DefinedContributionPlanUnvestedEmployerMatchingContributionsAmountToBeAmortized
Namespace Prefix: | gern_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- 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
Name: | gern_DefinedContributionPlanVestingPeriodOfMatchingContributions
Namespace Prefix: | gern_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- References No definition available. + Details
Name: | us-gaap_DefinedContributionPensionAndOtherPostretirementPlansDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration