Summary of Significant Accounting Policies - Narrative (Details)	9 Months Ended
Sep. 30, 2018
Marketable Securities
Minimum original maturity period of marketable securities	90 days

Represents the minimum original maturity period of marketable securities.

No definition available.

Name:	mrtx_MinimumOriginalMaturityPeriodOfMarketableSecurities
Namespace Prefix:	mrtx_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration