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FAIR VALUE MEASUREMENTS - CREDIT RISK (Details) | 12 Months Ended
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Dec. 31, 2016 item
Credit Risk |
Number of financial institutions with which cash, restricted cash, cash equivalents and marketable securities are placed | 4

X
- Definition Represents the number of financial institutions with which cash, restricted cash, cash equivalents and marketable securities placed. + References No definition available. + Details
Name: | gern_NumberOfFinancialInstitutionsWithWhichCashCashEquivalentsAndMarketableSecuritiesArePlaced
Namespace Prefix: | gern_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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