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COMMITMENTS AND CONTINGENCIES - Severance Plan (Details) | 12 Months Ended
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Dec. 31, 2017
Severance Plan |
Period within which employee is terminated by entity without cause following a change of control | 12 months
Period within which no comparable employment is offered by the entity following a change of control | 30 days
Period within which employee resigns following a change of control due to material change in terms of employment | 12 months
Minimum |
Severance Plan |
Period of base salary to be considered for severance payments | 2 months
Maximum |
Severance Plan |
Period of base salary to be considered for severance payments | 18 months

X
- Definition Loss contingency period of current base salary to be paid under severance plan. + References No definition available. + Details
Name: | gern_LossContingencyPeriodOfCurrentBaseSalaryToBePaidUnderSeverancePlan
Namespace Prefix: | gern_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the number of months following a change in control when an employee is terminated without cause to be classified as a triggering event for severance payments. + References No definition available. + Details
Name: | gern_LossContingencyTriggeringEventEmployeeTerminationWithoutCauseFollowingChangeOfControlPeriod
Namespace Prefix: | gern_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the period following change of control within which an employee is not offered comparable employment (new or continuing) by the entity or its successor or acquired and to be considered as triggering event for severance payments. + References No definition available. + Details
Name: | gern_LossContingencyTriggeringEventPeriodFollowingChangeOfControlDuringWhichNoComparableEmploymentIsOfferedByEntity
Namespace Prefix: | gern_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the period within which employee resigns following a change of control due to material change in terms of employment and to be considered as triggering event for severance payments. + References No definition available. + Details
Name: | gern_LossContingencyTriggeringEventPeriodFollowingChangeOfControlWithinWhichEmployeeResignsDueToMaterialChangeInTermsOfEmployment
Namespace Prefix: | gern_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Severance plan. + References No definition available. + Details
Name: | gern_SeverancePlanLineItems
Namespace Prefix: | gern_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |