License, Collaboration, and Funding Agreements - Genzyme Agreement (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Payment on obligation under license agreement	$ 900,000
Genzyme Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payments	$ 25,000,000.0
Payment on obligation under license agreement		$ 0	$ 0