Significant Agreements - Roche License Agreement (Details) - USD ($)	1 Months Ended			3 Months Ended		9 Months Ended		12 Months Ended
	Jun. 30, 2017	May 31, 2014	Feb. 28, 2014	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2019
License Agreement [Line Items]
Collaboration revenue				$ 200,000	$ 200,000	$ 400,000	$ 600,000
Accounts receivable				22,000		$ 22,000			$ 68,000
Collaborative Arrangement
License Agreement [Line Items]
License agreement, portion of revenue and expenses attributable to company, percentage						30.00%
Roche
License Agreement [Line Items]
Revenue, remaining performance obligation, amount				0		$ 0			0
License agreement, development services, payment term						45 days
License agreement, milestone payments, payment term						45 days
Roche | Collaborative Arrangement
License Agreement [Line Items]
Collaboration revenue, license, upfront payment			$ 30,000,000.0
Milestone payment received, clinical milestone		$ 15,000,000.0
Milestone achievement, clinical milestone	$ 30,000,000.0
License agreement, potential payment upon achievement of development, regulatory and various first commercial sales milestones						$ 350,000,000.0
License agreement, potential payment for achievement of non U.S. commercial sales milestones						$ 175,000,000.0
License agreement, cost allocation, percentage						100.00%
License agreement, portion of revenue and expenses attributable to company, percentage						70.00%
License agreement, potential alternative commercial sales milestones						$ 155,000,000.0
License agreement, initial transaction price				55,100,000		55,100,000
Cost sharing payments recognized as research and development expense				3,800,000	$ 2,700,000	12,300,000	$ 6,700,000
Accounts receivable				2,000		2,000			$ 2,000
Roche | Development Costs Reimbursement
License Agreement [Line Items]
Development reimbursement								$ 1,000,000.0
Roche | Research Reimbursement
License Agreement [Line Items]
Revenue, remaining performance obligation, amount				0		0
Royalty Bearing License | Roche
License Agreement [Line Items]
Allocated consideration to performance obligations				48,900,000		48,900,000
ASC 605 allocated consideration to deliverables				35,600,000		35,600,000
Royalty Bearing License | Roche | Collaborative Arrangement
License Agreement [Line Items]
License agreement, initial transaction price				45,000,000.0		45,000,000.0
IND and Development Services | Roche
License Agreement [Line Items]
Allocated consideration to performance obligations				4,600,000		4,600,000
ASC 605 allocated consideration to deliverables				3,300,000		3,300,000
IND and Development Services | Roche | Collaborative Arrangement
License Agreement [Line Items]
License agreement, initial transaction price				9,100,000		9,100,000
Supply Services | Roche
License Agreement [Line Items]
Allocated consideration to performance obligations				600,000		600,000
ASC 605 allocated consideration to deliverables				400,000		400,000
Supply Services | Roche | Collaborative Arrangement
License Agreement [Line Items]
License agreement, initial transaction price				1,100,000		1,100,000
Clinical Product Supply | Roche
License Agreement [Line Items]
Allocated consideration to performance obligations				1,100,000		1,100,000
ASC 605 allocated consideration to deliverables				$ 800,000		$ 800,000