Equity offerings (Details) $ in Millions	1 Months Ended
Mar. 31, 2017 USD ($)
At The Market Equity Offering Program
At-the-market equity offering program
Maximum value of common stock allowed to be sold	$ 35.0

The maximum value of common stock able to be offered and sold through the market equity offering program.

No definition available.

Name:	vstm_MaximumValueOfCommonStockOfferedThroughMarketEquityOfferingProgram
Namespace Prefix:	vstm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration