License, collaboration and commercial agreements - Secura - Contract asset (Details) - License and Collaboration Agreement $ in Thousands	9 Months Ended
Sep. 30, 2021 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additions	$ 152
Reclassification to receivable	(22)
Balance at the end of period	130
Secura
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additions	152
Reclassification to receivable	(22)
Balance at the end of period	130
Prepaid Expenses and Other Current Assets \| Secura
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Balance at the end of period	$ 100

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.

+ Details

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

- Definition

Amount, after allowance for credit loss, of right to consideration in exchange for good or service transferred to customer when right is conditioned on something other than passage of time.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130549-203045

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121556615&loc=SL49130531-203044

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=121556615&loc=SL49130533-203044

+ Details

Name:	us-gaap_ContractWithCustomerAssetNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of increase from purchase of right to consideration in exchange for good or service transferred to customer when right is conditioned on passage of time.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 11
-Subparagraph (h)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919244-210447

+ Details

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

- Definition

Amount of decrease in right to consideration in exchange for good or service transferred to customer when right is conditioned on something other than passage of time from transfer to receivable due to right to consideration becoming unconditional.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130551-203045

+ Details

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

- Details

Name:	us-gaap_TypeOfArrangementAxis=vstm_LicenseAndCollaborationAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=vstm_SecuraBioInc.Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_PrepaidExpensesAndOtherCurrentAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: