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CONDENSED STATEMENTS OF CASH FLOWS DATA (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Supplemental Operating Activities:
Issuance of common stock for 401(k) matching contributions		$ 313us-gaap_StockIssuedDuringPeriodValueEmployeeBenefitPlan
Reclassification of deposits to other current assets		4gern_ReclassificationOfDepositsToOtherCurrentAssets
Supplemental Investing Activities:
Net unrealized gain (loss) on marketable securities	$ 67us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss	$ (76)us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss

X

- Definition

Represents the amount of deposits that are expected to be returned within one year and are therefore reclassified to other current assets.

+ References

No definition available.

+ Details

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Namespace Prefix:	gern_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

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- Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

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- Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

This item represents the pretax change in net unrealized holding gain (loss) on available-for-sale securities that has been included in accumulated other comprehensive income, a separate component of shareholders' equity, during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 9

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Value of shares issued during the period to an employee benefit plan, such as a defined contribution or defined benefit plan.

+ References

No definition available.

+ Details

Name:	us-gaap_StockIssuedDuringPeriodValueEmployeeBenefitPlan
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration