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FAIR VALUE MEASUREMENTS (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Included in cash and cash equivalents
Amortized Cost	$ 40,342us-gaap_CashEquivalentsAtCarryingValue	$ 10,285us-gaap_CashEquivalentsAtCarryingValue
Estimated Fair Value	40,342us-gaap_CashAndCashEquivalentsFairValueDisclosure	10,285us-gaap_CashAndCashEquivalentsFairValueDisclosure
Restricted cash
Amortized Cost	266us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue	795us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue
Estimated Fair Value	266us-gaap_RestrictedInvestmentsAtFairValue	795us-gaap_RestrictedInvestmentsAtFairValue
Marketable securities
Amortized Cost	127,661us-gaap_AvailableForSaleSecuritiesAmortizedCost	52,248us-gaap_AvailableForSaleSecuritiesAmortizedCost
Gross Unrealized Gains	16gern_GrossUnrealizedGainsAvailableForSaleSecurities	5gern_GrossUnrealizedGainsAvailableForSaleSecurities
Gross Unrealized Losses	(100)gern_GrossUnrealizedLossesAvailableForSaleSecurities	(19)gern_GrossUnrealizedLossesAvailableForSaleSecurities
Estimated Fair Value	127,577us-gaap_AvailableForSaleSecurities	52,234us-gaap_AvailableForSaleSecurities
Money market funds
Included in cash and cash equivalents
Amortized Cost	40,342us-gaap_CashEquivalentsAtCarryingValue / us-gaap_InvestmentTypeAxis = us-gaap_MoneyMarketFundsMember	8,079us-gaap_CashEquivalentsAtCarryingValue / us-gaap_InvestmentTypeAxis = us-gaap_MoneyMarketFundsMember
Estimated Fair Value	40,342us-gaap_CashAndCashEquivalentsFairValueDisclosure / us-gaap_InvestmentTypeAxis = us-gaap_MoneyMarketFundsMember	8,079us-gaap_CashAndCashEquivalentsFairValueDisclosure / us-gaap_InvestmentTypeAxis = us-gaap_MoneyMarketFundsMember
Corporate notes
Included in cash and cash equivalents
Amortized Cost		2,206us-gaap_CashEquivalentsAtCarryingValue / us-gaap_InvestmentTypeAxis = us-gaap_CorporateNoteSecuritiesMember
Estimated Fair Value		2,206us-gaap_CashAndCashEquivalentsFairValueDisclosure / us-gaap_InvestmentTypeAxis = us-gaap_CorporateNoteSecuritiesMember
Certificates of deposit
Restricted cash
Amortized Cost	266us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue / us-gaap_InvestmentTypeAxis = us-gaap_CertificatesOfDepositMember	795us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue / us-gaap_InvestmentTypeAxis = us-gaap_CertificatesOfDepositMember
Estimated Fair Value	266us-gaap_RestrictedInvestmentsAtFairValue / us-gaap_InvestmentTypeAxis = us-gaap_CertificatesOfDepositMember	795us-gaap_RestrictedInvestmentsAtFairValue / us-gaap_InvestmentTypeAxis = us-gaap_CertificatesOfDepositMember
Government-sponsored enterprise securities (due in less than 1 year)
Marketable securities
Amortized Cost	401us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = gern_USGovernmentSponsoredEnterprisesDebtSecuritiesDueInLessThanOneYearMember	7,369us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = gern_USGovernmentSponsoredEnterprisesDebtSecuritiesDueInLessThanOneYearMember
Gross Unrealized Gains		1gern_GrossUnrealizedGainsAvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = gern_USGovernmentSponsoredEnterprisesDebtSecuritiesDueInLessThanOneYearMember
Gross Unrealized Losses	(1)gern_GrossUnrealizedLossesAvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = gern_USGovernmentSponsoredEnterprisesDebtSecuritiesDueInLessThanOneYearMember	(1)gern_GrossUnrealizedLossesAvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = gern_USGovernmentSponsoredEnterprisesDebtSecuritiesDueInLessThanOneYearMember
Estimated Fair Value	400us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = gern_USGovernmentSponsoredEnterprisesDebtSecuritiesDueInLessThanOneYearMember	7,369us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = gern_USGovernmentSponsoredEnterprisesDebtSecuritiesDueInLessThanOneYearMember
Government-sponsored enterprise securities (due in 1 to 2 years)
Marketable securities
Amortized Cost	6,556us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = gern_GovernmentSponsoredEnterpriseSecuritiesDueInOneToTwoYearsMember
Gross Unrealized Losses	(7)gern_GrossUnrealizedLossesAvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = gern_GovernmentSponsoredEnterpriseSecuritiesDueInOneToTwoYearsMember
Estimated Fair Value	6,549us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = gern_GovernmentSponsoredEnterpriseSecuritiesDueInOneToTwoYearsMember
Commercial paper (due in less than 1 year)
Marketable securities
Amortized Cost	10,985us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = gern_CommercialPaperDueInLessThanOneYearMember	5,496us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = gern_CommercialPaperDueInLessThanOneYearMember
Gross Unrealized Gains	14gern_GrossUnrealizedGainsAvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = gern_CommercialPaperDueInLessThanOneYearMember	3gern_GrossUnrealizedGainsAvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = gern_CommercialPaperDueInLessThanOneYearMember
Estimated Fair Value	10,999us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = gern_CommercialPaperDueInLessThanOneYearMember	5,499us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = gern_CommercialPaperDueInLessThanOneYearMember
Corporate notes (due in less than 1 year)
Marketable securities
Amortized Cost	97,307us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = gern_CorporateNoteSecuritiesDueInLessThanOneYearMember	39,383us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = gern_CorporateNoteSecuritiesDueInLessThanOneYearMember
Gross Unrealized Gains	2gern_GrossUnrealizedGainsAvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = gern_CorporateNoteSecuritiesDueInLessThanOneYearMember	1gern_GrossUnrealizedGainsAvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = gern_CorporateNoteSecuritiesDueInLessThanOneYearMember
Gross Unrealized Losses	(63)gern_GrossUnrealizedLossesAvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = gern_CorporateNoteSecuritiesDueInLessThanOneYearMember	(18)gern_GrossUnrealizedLossesAvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = gern_CorporateNoteSecuritiesDueInLessThanOneYearMember
Estimated Fair Value	97,246us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = gern_CorporateNoteSecuritiesDueInLessThanOneYearMember	39,366us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = gern_CorporateNoteSecuritiesDueInLessThanOneYearMember
Corporate notes (due in 1 to 2 years)
Marketable securities
Amortized Cost	12,412us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = gern_CorporateNoteSecuritiesDueInOneToTwoYearsMember
Gross Unrealized Losses	(29)gern_GrossUnrealizedLossesAvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = gern_CorporateNoteSecuritiesDueInOneToTwoYearsMember
Estimated Fair Value	$ 12,383us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = gern_CorporateNoteSecuritiesDueInOneToTwoYearsMember

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Amount of short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

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The aggregate value of all restricted investments.

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