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STOCKHOLDERS' EQUITY - 401(k) PLAN (Details)	12 Months Ended
	Dec. 31, 2013
401(k) Plan
Period of vesting of matching contributions	4 years

X

- Definition

Represents the period over which matching contributions invested in common stock will vest ratably for each year of service completed by the employee.

+ References

No definition available.

+ Details

Name:	gern_DefinedContributionPlanVestingPeriodOfMatchingContributions
Namespace Prefix:	gern_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- References

No definition available.

+ Details

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