Shareholders' Equity (Deficit) - Narrative (Details) $ / shares in Units, $ in Thousands			1 Months Ended			2 Months Ended	12 Months Ended
	Nov. 01, 2016 USD ($) $ / shares shares	Oct. 18, 2016	Apr. 30, 2017 shares	Aug. 31, 2016 shares	Jun. 30, 2016 shares	Mar. 31, 2016 USD ($)	Mar. 31, 2018 USD ($) $ / shares shares	Apr. 30, 2017 shares	Mar. 31, 2017 USD ($) $ / shares shares	Feb. 02, 2016 class
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of classes of shares | class										1
Common shares authorized (in shares)							564,111,242		564,111,242
Common shares par value (in USD per share) | $ / shares							$ 0.000017727		$ 0.000017727
Reverse stock split ratio		0.5641
Shares sold, price (in USD per share) | $ / shares									$ 15.00
Gross proceeds on sale of shares | $							$ 1,857		$ 199,964
Net proceeds on sale of shares | $						$ 0	$ 0		$ 202,275
Options granted (in shares)			32,500				2,338,116		1,525,857
Shares issued to settle the warrant liability to Takeda (in shares)								2,343,624	2,339,192
Initial Public Offering
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares sold (in shares)	14,500,000
Shares sold, price (in USD per share) | $ / shares	$ 15.00
Gross proceeds on sale of shares | $	$ 217,500
Net proceeds on sale of shares | $	200,000
Underwriting discounts and commissions | $	15,200
Offering costs | $	$ 2,300
Shares issued to settle the warrant liability to Takeda (in shares)									1,977,269
Restricted Share Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards granted (in shares)					1,100,000				1,128,222
Shares issued to settle the warrant liability to Takeda (in shares)									153,846
Employee and Non-employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted (in shares)				600,000
Shares issued to settle the warrant liability to Takeda (in shares)							4,432		208,077