Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Mar. 31, 2017	Apr. 29, 2016
Liabilities:
Takeda warrant liability			$ 5,400
Fair Value, Measurements, Recurring
Liabilities:
Takeda warrant liability	$ 0	$ 52
Total liabilities at fair value	0	52
Fair Value, Measurements, Recurring | Level 1
Liabilities:
Takeda warrant liability	0	0
Total liabilities at fair value	0	0
Fair Value, Measurements, Recurring | Level 2
Liabilities:
Takeda warrant liability	0	0
Total liabilities at fair value	0	0
Fair Value, Measurements, Recurring | Level 3
Liabilities:
Takeda warrant liability	0	52
Total liabilities at fair value	$ 0	$ 52