Regeneron License and Collaboration Arrangement - Additional Information (Details) - Regeneron Pharmaceuticals, Inc. - USD ($) $ in Millions		3 Months Ended			6 Months Ended	9 Months Ended		12 Months Ended
	Jul. 29, 2016	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Non refundable upfront payment received	$ 25.0
Additional payment for research funding received	$ 20.0							$ 5.0	$ 5.0
License agreement termination description	12 years
Contract research term period	5 years
Upfront payment	$ 25.0					$ 25.0
Licence agreement cumulative revenue recognised			$ 5.0		$ 10.0	25.6	$ 11.1
License agreement cumulative revenue reduction recognised				$ 6.6
Increase in estimated transaction price			$ 10.0
License and collaboration revenue		$ 3.0		$ (7.0)		12.5	(1.0)
Contract liability						$ 19.4	$ 23.9
Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Licence agreement additional amount payable of option exercise fees	$ 100.0