Revenue - Additional Information (Details) - USD ($)		1 Months Ended	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Feb. 28, 2019	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation Of Revenue [Line Items]
Revenue from collaboration agreement			$ 39,467,000	$ 26,299,000	$ 116,414,000	$ 68,197,000
Revenue recognized	$ 36,700,000				(73,065,000)
Development Revenue
Disaggregation Of Revenue [Line Items]
Revenue from collaboration agreement			17,361,000	15,023,000	60,579,000	39,467,000
MRD Development Agreements
Disaggregation Of Revenue [Line Items]
Revenue from collaboration agreement					3,200,000
Milestone revenue recognized			1,500,000	2,500,000	10,000,000.0	2,500,000
Revenue recognized			1,900,000		11,400,000
MRD Development Agreements | Maximum
Disaggregation Of Revenue [Line Items]
Additional milestone payment receivable	333,500,000		333,500,000		333,500,000
MRD Development Agreements | Development Revenue
Disaggregation Of Revenue [Line Items]
Revenue from collaboration agreement			300,000		300,000
Revenue recognized				2,600,000		3,100,000
Genentech Collaboration Agreement
Disaggregation Of Revenue [Line Items]
Revenue from collaboration agreement			15,000,000.0	$ 12,300,000	47,800,000	$ 35,900,000
Non-refundable upfront payments received		$ 300,000,000.0			$ 300,000,000.0
Genentech Collaboration Agreement | Maximum
Disaggregation Of Revenue [Line Items]
Revenue recognition expected period					8 years
Genentech Collaboration Agreement | Maximum | Regulatory Milestones
Disaggregation Of Revenue [Line Items]
Expected revenue through milestone payments	75,000,000.0		75,000,000.0		$ 75,000,000.0
Genentech Collaboration Agreement | Maximum | Development Milestones
Disaggregation Of Revenue [Line Items]
Expected revenue through milestone payments	300,000,000.0		300,000,000.0		300,000,000.0
Genentech Collaboration Agreement | Maximum | Commercial Milestones
Disaggregation Of Revenue [Line Items]
Expected revenue through milestone payments	1,430,000,000.0		1,430,000,000.0		1,430,000,000.0
Genentech Collaboration Agreement | Minimum
Disaggregation Of Revenue [Line Items]
Expected revenue through milestone payments	$ 1,800,000		$ 1,800,000		$ 1,800,000
Revenue recognition expected period					7 years