Name:	grna_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityWarrantsAssumedInBusinessCombination
Namespace Prefix:	grna_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Fair value, measurement with unobservable inputs reconciliation, recurring basis, liability warrants exercised in business combination.

No definition available.

Name:	grna_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityWarrantsExercisedInBusinessCombination
Namespace Prefix:	grna_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Fair value, measurement with unobservable inputs reconciliation, recurring basis, liability warrants reclassified to equity.

No definition available.

Name:	grna_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityWarrantsReclassifiedToEquity
Namespace Prefix:	grna_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

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.

No definition available.

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

Amount of gain (loss) recognized in income from liability measured at fair value on recurring basis using unobservable input (level 3).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
-URI https://asc.fasb.org/extlink&oid=126976982&loc=d3e19207-110258

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

Fair value of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI https://asc.fasb.org/extlink&oid=126976982&loc=d3e19207-110258