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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2018
License Agreement [Line Items]
Collaboration revenue				$ 205,000	$ 255,000	$ 558,000	$ 761,000
Collaborative Arrangement [Member]
License Agreement [Line Items]
Licensing Agreement, Portion of Revenue and Expenses Attributable to Company, Percentage						30.00%
Roche [Member]
License Agreement [Line Items]
Milestone Achievement, Clinical Milestone	$ 30,000,000
Milestone Method, Portion of Milestone Amount Recognized as Offset to R&D Expense								$ 3,400,000
Collaboration Revenue, Clinical Milestone								$ 26,600,000
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 [Member] | Royalty Bearing License [Member]
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
Roche [Member] | IND and Development Services [Member]
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
Roche [Member] | Supply Services [Member]
License Agreement [Line Items]
Allocated Consideration to Performance Obligations				600,000		600,000
ASC 605 Allocated Consideration to Deliverables				400,000		400,000
Roche [Member] | Clinical Product Supply [Member]
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
Roche [Member] | Collaborative Arrangement [Member]
License Agreement [Line Items]
Collaboration Revenue, License, Upfront Payment			$ 30,000,000
Milestone Payment Received, Clinical Milestone		$ 15,000,000
License Agreement, Potential Payment Upon Achievement of Development, Regulatory and Various First Commercial Sales Milestones						350,000,000
License Agreement, Potential Payment for Achievement of non U.S. Commercial Sales Milestones						$ 175,000,000
Licensing Agreement, Cost Allocation, Percentage						100.00%
Licensing Agreement, Portion of Revenue and Expenses Attributable to Company, Percentage						70.00%
License Agreement, Potential Alternative Commercial Sales Milestones						$ 155,000,000
License Agreement, Initial Transaction Price				55,100,000		55,100,000
Cost sharing payments recognized as research and development expense				2,700,000	3,100,000	6,700,000	9,500,000
Accounts Receivable, after Allowance for Credit Loss, Current				42,000		42,000			$ 2,000
Roche [Member] | Collaborative Arrangement [Member] | Royalty Bearing License [Member]
License Agreement [Line Items]
License Agreement, Initial Transaction Price				45,000,000		45,000,000
Roche [Member] | Collaborative Arrangement [Member] | IND and Development Services [Member]
License Agreement [Line Items]
License Agreement, Initial Transaction Price				9,100,000		9,100,000
Roche [Member] | Collaborative Arrangement [Member] | Supply Services [Member]
License Agreement [Line Items]
License Agreement, Initial Transaction Price				1,100,000		1,100,000
Roche [Member] | Development Costs Reimbursement [Member]
License Agreement [Line Items]
Development Reimbursement					$ 300,000		$ 800,000
Roche [Member] | Research Reimbursement [Member]
License Agreement [Line Items]
Revenue, Remaining Performance Obligation, Amount				$ 0		$ 0