Collaboration and Licensing Agreements (Details) - Gilead Collaboration and License Agreement $ in Millions	1 Months Ended	3 Months Ended
	Jun. 30, 2018 USD ($) item	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)
Collaboration and Licensing Agreements
Number of preclinical research programs | item	2
Non-refundable, upfront payment	$ 10.0
Upfront payment recorded as revenue		$ 0.4	$ 0.9
Revenue recognized from milestone payments		4.0
Liability in deferred revenues		$ 5.5
Percentage of upfront payment included in deferred revenue expected to be recognized in 2020		68.00%
Percentage of upfront payment included in deferred revenue expected to be recognized in 2021		28.00%
Percentage of upfront payment included in deferred revenue expected to be recognized in 2022		4.00%
Payment term (in days)		60 days
Revenue recognized from cost reimbursements		$ 2.7	$ 1.3
Upfront payment recorded as revenue		0.6
Contract asset relating to the sublicense payment		0.3		$ 0.3
Contract liability relating to sublicense payment		0.3		$ 0.3
Development Milestones
Collaboration and Licensing Agreements
Total milestone amount		280.0
Commercial Milestones
Collaboration and Licensing Agreements
Total milestone amount		$ 100.0