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STOCKHOLDERS' EQUITY (Details 2) (USD $) | 12 Months Ended
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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 |  |

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- Definition Represents the minimum exercise price as a percentage of fair market value for employees having more than 10 % outstanding common stock. + References No definition available. + Details
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- Definition Represents the minimum percentage of ownership required for granting of options at a price not less than 110 percent of the fair market value of common stock. + References No definition available. + Details
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- Definition The number of options granted with an exercise price below or greater than fair market value of the company's common stock on the date of grant. + References No definition available. + Details
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- Definition Represents the number of options to be granted to purchase shares in each year during the optionee's service on the Board of Directors. + References No definition available. + Details
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- Definition Represents the number of options to be granted to purchase shares upon appointment to Board of Directors. + References No definition available. + Details
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- Definition Represents the maximum expiration term of options granted to employees having more than 10 % outstanding common stock. + References No definition available. + Details
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- Definition Description of award terms as to how many shares or portion of an award are no longer contingent on satisfaction of either a service condition, market condition or a performance condition, thereby giving the employee the legal right to convert the award to shares, shown as a percentage. + References No definition available. + Details
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- Definition Disclosure of the aggregate tax benefit realized from the exercise of stock options and the conversion of similar instruments during the annual period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The cash inflow associated with the amount received from holders exercising their stock options. This item inherently excludes any excess tax benefit, which the entity may have realized and reported separately. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Financing Activities -URI http://asc.fasb.org/extlink&oid=6513228 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 14 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3255-108585 + Details
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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iv)(3) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iv)(3) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Gross number of share options (or share units) granted during the period. + References No definition available. + Details
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- Definition Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Number of options outstanding, including both vested and non-vested options. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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