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

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

No definition available.

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