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STOCKHOLDERS' EQUITY - 401(k) PLAN (Details) | 12 Months Ended
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Dec. 31, 2013
401(k) Plan |
Period of vesting of matching contributions | 4 years

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