Accrued Expenses (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Mar. 31, 2019
Payables and Accruals [Abstract]
Accrued R&D expenses	$ 36,620	$ 46,947
Accrued compensation-related expenses	3,800	5,024
Accrued professional service fees	1,053	370
Accrued other expenses	2,907	1,273
Total accrued expenses	$ 44,380	$ 53,614