Accrued Expenses (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Mar. 31, 2018
Payables and Accruals [Abstract]
Accrued R&D expenses	$ 34,145	$ 25,988
Accrued compensation-related expenses	2,423	2,792
Accrued professional fees	624	566
Accrued other expenses	1,541	919
Total accrued expenses	$ 38,733	$ 30,265