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FAIR VALUE MEASUREMENTS - CREDIT RISK (Details)	12 Months Ended
	Dec. 31, 2016 item
Credit Risk
Number of financial institutions with which cash, restricted cash, cash equivalents and marketable securities are placed	4

X

- Definition

Represents the number of financial institutions with which cash, restricted cash, cash equivalents and marketable securities placed.

+ References

No definition available.

+ Details

Name:	gern_NumberOfFinancialInstitutionsWithWhichCashCashEquivalentsAndMarketableSecuritiesArePlaced
Namespace Prefix:	gern_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

X

- References

No definition available.

+ Details

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