Maximum number of months of an employee's then current base salary to be paid as severance under severance plan.

No definition available.

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

- Definition

Minimum number of months of an employee's then current base salary to be paid as severance under severance plan.

+ References

No definition available.

+ Details

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

- Definition

Represents the number of months following a change in control when an employee is terminated without cause to be classified as a triggering event for severance payments.

+ References

No definition available.

+ Details

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

- Definition

Represents the period following change of control within which an employee is not offered comparable employment (new or continuing) by the entity or its successor or acquired and to be considered as triggering event for severance payments.

+ References

No definition available.

+ Details

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

- Definition

Represents the period within which employee resigns following a change of control due to material change in terms of employment and to be considered as triggering event for severance payments.

+ References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Definition

Amount of required minimum rental payments for leases having an initial or remaining non-cancelable letter-terms in excess of one year.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 10
-Section 55
-Paragraph 40
-Subparagraph (Note 3)
-URI http://asc.fasb.org/extlink&oid=6584154&loc=d3e38371-112697

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 20
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6453985&loc=d3e41502-112717

+ Details

Name:	us-gaap_OperatingLeasesFutureMinimumPaymentsDue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Rental expense for the reporting period incurred under operating leases, including minimum and any contingent rent expense, net of related sublease income.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6453985&loc=d3e41499-112717

+ Details

Name:	us-gaap_OperatingLeasesRentExpenseNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_LossContingenciesByNatureOfContingencyAxis=us-gaap_EmployeeSeveranceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

COMMITMENTS AND CONTINGENCIES (Details) - USD ($)	12 Months Ended
COMMITMENTS AND CONTINGENCIES (Details) - USD ($)	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
COMMITMENTS AND CONTINGENCIES.
Future minimum payments under operating lease	$ 1,386,000
Rent expense under operating leases	$ 878,000	$ 936,000	$ 1,422,000
Severance Plan
COMMITMENTS AND CONTINGENCIES.
Period within which employee is terminated by entity without cause following a change of control	12 months
Period within which no comparable employment is offered by the entity following a change of control	30 days
Period within which employee resigns following a change of control due to material change in terms of employment	12 months
Severance Plan \| Minimum
COMMITMENTS AND CONTINGENCIES.
Period of base salary to be considered for severance payments	2 months
Severance Plan \| Maximum
COMMITMENTS AND CONTINGENCIES.
Period of base salary to be considered for severance payments	18 months