Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Mar. 31, 2017
Payables and Accruals [Abstract]
Accrued research and development expenses	$ 18,287	$ 9,737
Accrued compensation-related expenses	1,838	797
Accrued legal expenses	133	481
Accrued other expenses	695	963
Total accrued expenses	$ 20,953	$ 11,978