Accrued Expenses (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Mar. 31, 2018
Payables and Accruals [Abstract]
Accrued R&D expenses	$ 35,446	$ 25,988
Accrued compensation-related expenses	1,229	2,792
Accrued professional fees	551	566
Accrued other expenses	1,079	919
Total accrued expenses	$ 38,305	$ 30,265