Accrued Expenses (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Mar. 31, 2020
Payables and Accruals [Abstract]
Accrued R&D expenses	$ 17,911	$ 15,500
Accrued compensation-related expenses	12,330	9,309
Accrued commercial expenses	5,898	818
Accrued professional service fees	1,068	1,126
Accrued other expenses	2,934	2,307
Accrued expenses	$ 40,141	$ 29,060