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FAIR VALUE MEASUREMENTS (Details 2) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Marketable securities with unrealized losses |  |
Less Than 12 Months - Estimated Fair Value | $ 111,321us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue | $ 41,007us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Less Than 12 Months - Gross Unrealized Losses | (100)gern_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsAggregateLosses | (19)gern_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsAggregateLosses
Total - Estimated Fair Value | 111,321us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue | 41,007us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
Total - Gross Unrealized Losses | (100)gern_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLoss | (19)gern_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLoss
Government-sponsored enterprise securities (due in less than 1 year) |  |
Marketable securities with unrealized losses |  |
Less Than 12 Months - Estimated Fair Value | 400us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = gern_USGovernmentSponsoredEnterprisesDebtSecuritiesDueInLessThanOneYearMember | 3,947us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = gern_USGovernmentSponsoredEnterprisesDebtSecuritiesDueInLessThanOneYearMember
Less Than 12 Months - Gross Unrealized Losses | (1)gern_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsAggregateLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = gern_USGovernmentSponsoredEnterprisesDebtSecuritiesDueInLessThanOneYearMember | (1)gern_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsAggregateLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = gern_USGovernmentSponsoredEnterprisesDebtSecuritiesDueInLessThanOneYearMember
Total - Estimated Fair Value | 400us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = gern_USGovernmentSponsoredEnterprisesDebtSecuritiesDueInLessThanOneYearMember | 3,947us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = gern_USGovernmentSponsoredEnterprisesDebtSecuritiesDueInLessThanOneYearMember
Total - Gross Unrealized Losses | (1)gern_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = gern_USGovernmentSponsoredEnterprisesDebtSecuritiesDueInLessThanOneYearMember | (1)gern_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = gern_USGovernmentSponsoredEnterprisesDebtSecuritiesDueInLessThanOneYearMember
Government-sponsored enterprise securities (due in 1 to 2 years) |  |
Marketable securities with unrealized losses |  |
Less Than 12 Months - Estimated Fair Value | 5,549us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = gern_GovernmentSponsoredEnterpriseSecuritiesDueInOneToTwoYearsMember |
Less Than 12 Months - Gross Unrealized Losses | (7)gern_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsAggregateLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = gern_GovernmentSponsoredEnterpriseSecuritiesDueInOneToTwoYearsMember |
Total - Estimated Fair Value | 5,549us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = gern_GovernmentSponsoredEnterpriseSecuritiesDueInOneToTwoYearsMember |
Total - Gross Unrealized Losses | (7)gern_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = gern_GovernmentSponsoredEnterpriseSecuritiesDueInOneToTwoYearsMember |
Corporate notes (due in less than 1 year) |  |
Marketable securities with unrealized losses |  |
Less Than 12 Months - Estimated Fair Value | 92,989us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = gern_CorporateNoteSecuritiesDueInLessThanOneYearMember | 37,060us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = gern_CorporateNoteSecuritiesDueInLessThanOneYearMember
Less Than 12 Months - Gross Unrealized Losses | (63)gern_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsAggregateLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = gern_CorporateNoteSecuritiesDueInLessThanOneYearMember | (18)gern_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsAggregateLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = gern_CorporateNoteSecuritiesDueInLessThanOneYearMember
Total - Estimated Fair Value | 92,989us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = gern_CorporateNoteSecuritiesDueInLessThanOneYearMember | 37,060us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = gern_CorporateNoteSecuritiesDueInLessThanOneYearMember
Total - Gross Unrealized Losses | (63)gern_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = gern_CorporateNoteSecuritiesDueInLessThanOneYearMember | (18)gern_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = gern_CorporateNoteSecuritiesDueInLessThanOneYearMember
Corporate notes (due in 1 to 2 years) |  |
Marketable securities with unrealized losses |  |
Less Than 12 Months - Estimated Fair Value | 12,383us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = gern_CorporateNoteSecuritiesDueInOneToTwoYearsMember |
Less Than 12 Months - Gross Unrealized Losses | (29)gern_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsAggregateLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = gern_CorporateNoteSecuritiesDueInOneToTwoYearsMember |
Total - Estimated Fair Value | 12,383us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = gern_CorporateNoteSecuritiesDueInOneToTwoYearsMember |
Total - Gross Unrealized Losses | $ (29)gern_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = gern_CorporateNoteSecuritiesDueInOneToTwoYearsMember |

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