Revenue from Contracts with Customers - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2021
Disaggregation Of Revenue [Line Items]
Total revenues related to license and collaboration agreement for Diroximel fumarate	$ 223,102	$ 225,150
Minimum
Disaggregation Of Revenue [Line Items]
Manufacturing Process period	10 days
Maximum
Disaggregation Of Revenue [Line Items]
Manufacturing Process period	56 days
Biogen
Disaggregation Of Revenue [Line Items]
Total revenues related to license and collaboration agreement for Diroximel fumarate	$ 13,900	17,500
Research and development revenue
Disaggregation Of Revenue [Line Items]
Total revenues related to license and collaboration agreement for Diroximel fumarate	$ 14,706	$ 18,707
Research and development revenue | Biogen | Forecast
Disaggregation Of Revenue [Line Items]
Additional revenue expected			$ 77,600