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CONDENSED STATEMENTS OF CASH FLOWS DATA (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Supplemental Operating Activities: |  |
Issuance of common stock for 401(k) matching contributions |  | $ 313 us-gaap_StockIssuedDuringPeriodValueEmployeeBenefitPlan
Reclassification of deposits to other current assets |  | 4 gern_ReclassificationOfDepositsToOtherCurrentAssets
Supplemental Investing Activities: |  |
Net unrealized gain (loss) on marketable securities | $ 67 us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss | $ (76)us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss

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- Definition Represents the amount of deposits that are expected to be returned within one year and are therefore reclassified to other current assets. + References No definition available. + Details
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Namespace Prefix: | gern_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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

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

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- Definition This item represents the pretax change in net unrealized holding gain (loss) on available-for-sale securities that has been included in accumulated other comprehensive income, a separate component of shareholders' equity, during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Value of shares issued during the period to an employee benefit plan, such as a defined contribution or defined benefit plan. + References No definition available. + Details
Name: | us-gaap_StockIssuedDuringPeriodValueEmployeeBenefitPlan
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration