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STOCKHOLDERS' EQUITY (Details 2) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
STOCKHOLDERS' EQUITY      
Number of options granted with exercise price other than fair market value of Company's common stock 0gern_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfOptionsGrantedWithExercisePriceBelowOrGreaterThanFairValueAtDateOfGrant 0gern_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfOptionsGrantedWithExercisePriceBelowOrGreaterThanFairValueAtDateOfGrant 0gern_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfOptionsGrantedWithExercisePriceBelowOrGreaterThanFairValueAtDateOfGrant
Shares Available For Grant      
Balance at the beginning of the period (in shares) 16,207,250us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant    
Options granted (in shares) (5,658,931)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross    
Awards granted (in shares) (59,330)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod    
Options canceled/forfeited (in shares) 3,613,577us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod    
Awards canceled/forfeited (in shares) 142,375us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod    
Balance at the end of the period (in shares) 13,384,883us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant 16,207,250us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant  
Number of Shares      
Balance at the beginning of the period (in shares) 15,576,216us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber    
Options granted (in shares) 5,658,931us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross    
Options exercised (in shares) (662,626)us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised    
Options canceled/forfeited (in shares) (3,613,577)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod    
Balance at the end of the period (in shares) 16,958,944us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber 15,576,216us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber  
Options exercisable at the end of the period (in shares) 9,129,576us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber 8,144,040us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber 10,410,194us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber
Options fully vested and expected to vest at the end of the period (in shares) 16,225,022us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber    
Weighted Average Exercise Price Per Share      
Balance at the beginning of the period (in dollars per share) $ 3.04us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice    
Options granted (in dollars per share) $ 4.85us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice    
Options exercised (in dollars per share) $ 1.94us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice    
Options canceled/forfeited (in dollars per share) $ 5.48us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice    
Balance at the end of the period (in dollars per share) $ 3.16us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice $ 3.04us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice  
Options exercisable at the end of the period (in dollars per share) $ 3.12us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice $ 4.26us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice $ 5.49us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice
Options fully vested and expected to vest at the end of the period (in dollars per share) $ 3.14us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice    
Weighted Average Remaining Contractual Life (In years)      
Balance at the end of the period 7 years 4 months 17 days    
Options exercisable at the end of the period 6 years 5 months 19 days    
Options fully vested and expected to vest at the end of the period 7 years 3 months 26 days    
Aggregate Intrinsic Value      
Balance at the end of the period (in dollars) $ 16,038,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue $ 33,798,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue  
Options exercisable at the end of the period (in dollars) 9,231,000us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1    
Options fully vested and expected to vest at the end of the period (in dollars) 15,546,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue    
Closing stock price (in dollars per share) $ 3.25us-gaap_SharePrice    
Total pretax intrinsic value of stock options exercised (in dollars) 989,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue 2,787,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue 100us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue
Cash received from exercise of options (in dollars) 1,286,000us-gaap_ProceedsFromStockOptionsExercised 6,567,000us-gaap_ProceedsFromStockOptionsExercised 1,000us-gaap_ProceedsFromStockOptionsExercised
Income tax benefit realized from stock options exercised $ 0us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitRealizedFromExerciseOfStockOptions    
2002 Plan      
STOCKHOLDERS' EQUITY      
Vesting period for specific portion of options 4 years    
Exercise price of options as a percentage of fair market value 100.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent
/ us-gaap_PlanNameAxis
= gern_EquityIncentivePlan2002Member		    
2002 Plan | Maximum      
STOCKHOLDERS' EQUITY      
Expiration term of options from date of grant 10 years    
2011 Plan      
STOCKHOLDERS' EQUITY      
Vesting period for specific portion of options 4 years    
Exercise price of options as a percentage of fair market value 100.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent
/ us-gaap_PlanNameAxis
= gern_IncentiveAwardPlan2011Member		    
Minimum percentage of ownership required for granting options at least 110% of fair market value of common stock 10.00%gern_ShareBasedCompensationArrangementByShareBasedPaymentAwardMinimumPercentageOfOwnershipRequiredForGrantingOptionsAtMinimum110PercentOfFairMarketValue
/ us-gaap_PlanNameAxis
= gern_IncentiveAwardPlan2011Member		    
Minimum exercise price as a percentage of fair market value for employees having more than 10 % outstanding common stock 110.00%gern_ShareBasedCompensationArrangementByShareBasedPaymentAwardMinimumExercisePriceOfOptionsGrantedToEmployeesWithOwnershipOfMoreThan10PercentageOfCommonStock
/ us-gaap_PlanNameAxis
= gern_IncentiveAwardPlan2011Member		    
Maximum expiration term of options granted to employees having more than 10 % outstanding common stock 5 years    
Shares Available For Grant      
Balance at the end of the period (in shares) 13,384,883us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
/ us-gaap_PlanNameAxis
= gern_IncentiveAwardPlan2011Member		    
2011 Plan | Maximum      
STOCKHOLDERS' EQUITY      
Expiration term of options from date of grant 10 years    
2011 Plan | First Director Option      
STOCKHOLDERS' EQUITY      
Vesting period for specific portion of options 3 years    
Options to be granted to purchase shares upon appointment (in shares) 70,000gern_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsToBeGrantedToPurchaseSharesUponAppointment
/ us-gaap_AwardTypeAxis
= gern_FirstDirectorOptionMember
/ us-gaap_PlanNameAxis
= gern_IncentiveAwardPlan2011Member		    
2011 Plan | Subsequent Director Option      
STOCKHOLDERS' EQUITY      
Vesting period for specific portion of options 1 year    
Options to be granted to purchase shares (in shares) 35,000gern_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsToBeGrantedToPurchaseShares
/ us-gaap_AwardTypeAxis
= gern_SubsequentDirectorOptionMember
/ us-gaap_PlanNameAxis
= gern_IncentiveAwardPlan2011Member		    
1996 Directors Plan      
STOCKHOLDERS' EQUITY      
Vesting period for specific portion of options 3 years    
Percentage of option vesting 100.00%gern_VestingRightsPercentage
/ us-gaap_PlanNameAxis
= gern_DirectorsStockOptionPlan1996Member		    
1996 Directors Plan | Maximum      
STOCKHOLDERS' EQUITY      
Expiration term of options from date of grant 10 years    
2006 Directors Plan      
Shares Available For Grant      
Awards canceled/forfeited (in shares) (860,058)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod
/ us-gaap_PlanNameAxis
= gern_DirectorsPlan2006Member		    
2006 Directors Plan | Maximum      
STOCKHOLDERS' EQUITY      
Expiration term of options from date of grant 10 years    
2006 Directors Plan | First Director Option      
STOCKHOLDERS' EQUITY      
Vesting period for specific portion of options 3 years    
2006 Directors Plan | Subsequent Director Option      
STOCKHOLDERS' EQUITY      
Vesting period for specific portion of options 1 year