Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Mar. 31, 2019
Payables and Accruals [Abstract]
Accrued R&D expenses	$ 30,941	$ 46,947
Accrued compensation-related expenses	8,666	5,024
Accrued professional service fees	1,446	370
Accrued other expenses	4,161	1,394
Total accrued expenses	$ 45,214	$ 53,735