Collaboration Agreements - Sanofi (Details) $ in Thousands	1 Months Ended		3 Months Ended		6 Months Ended		21 Months Ended
	Jul. 31, 2019 USD ($)	Nov. 30, 2018 USD ($) indication	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of indications | indication		3
Sales			$ 5,811	$ 4,098	$ 9,363	$ 8,209
Collaborative Arrangement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Funded percentage		30.00%
Sanofi | Collaborative Arrangement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Funded percentage		70.00%
Sanofi | Collaborative Arrangement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront payment		$ 125,000
Milestone payments upon achievement of certain clinical, regulatory and sales milestone events		1,100,000
Increase in transaction price							$ 23,500
Receivable			300		300		300	$ 1,200
Sanofi | Collaborative Arrangement | CNS Product
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Clinical milestone payment		215,000
Regulatory milestone payment		385,000
Sanofi | Collaborative Arrangement | Peripheral Product
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Clinical milestone payment		120,000
Regulatory milestone payment		175,000
Commercial milestone payments		$ 200,000
Sanofi | Collaborative Arrangement | Retained Activities
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Increase in transaction price			$ 300	$ 3,400	$ 800	$ 6,800	13,500
Sanofi | Collaborative Arrangement | Milestone Triggered
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Sales	$ 10,000						10,000
Sanofi | Collaborative Arrangement | Product
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Sales							$ 0