Shareholders' Equity (Deficit) - Narrative (Details) $ / shares in Units, $ in Millions			1 Months Ended					2 Months Ended	12 Months Ended
	Nov. 01, 2016 USD ($) $ / shares shares	Oct. 18, 2016	Apr. 30, 2019 USD ($) shares	Dec. 31, 2018 USD ($) shares	Apr. 30, 2018 USD ($) $ / shares shares	Oct. 31, 2017 USD ($) $ / shares shares	Apr. 30, 2017 shares	Aug. 31, 2018 USD ($) shares	Mar. 31, 2019 USD ($) $ / shares shares	Mar. 31, 2018 $ / shares shares	Apr. 30, 2017 shares	Mar. 31, 2017 shares	Jul. 31, 2018 $ / shares	Feb. 02, 2016 class
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of classes of stock | 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 issued to settle the warrant liability (in shares)										4,432	2,343,624	2,339,192
Options granted (in shares)									2,246,410	2,338,116		1,525,857
Initial Public Offering
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares sold (in shares)	14,500,000
Price of shares sold (in shares) | $ / shares	$ 15.00
Gross proceeds from sale of shares | $	$ 217.5
Net proceeds from sale of shares | $	200.0
Underwriting discounts and commissions | $	15.2
Offering costs | $	$ 2.3
Shares issued to settle the warrant liability (in shares)												1,977,269
Public Equity Offering
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares sold (in shares)								3,533,399
Price of shares sold (in shares) | $ / shares													$ 22.50
Net proceeds from sale of shares | $								$ 74.4
Underwriter's Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares sold (in shares)								200,065
Private Placement
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares sold (in shares)						138,361
Net proceeds from sale of shares | $						$ 2.0
Restricted Share Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued to settle the warrant liability (in shares)												153,846
Awards granted (in shares)												1,128,222
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued to settle the warrant liability (in shares)							32,500					208,077
Roivant Sciences, Ltd. | Private Placement
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares sold (in shares)					1,110,015
Price of shares sold (in shares) | $ / shares					$ 20.27
Gross proceeds from sale of shares | $					$ 22.5
Cowen and Company, LLC | Private Placement
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares sold (in shares)									3,970,129
Price of shares sold (in shares) | $ / shares									$ 21.91
Net proceeds from sale of shares | $									$ 84.1
Aggregate offering price | $					$ 100.0
Remaining capacity in offering program | $									$ 13.0
NovaQuest | Private Placement
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares sold (in shares)				1,082,977
Net proceeds from sale of shares | $				$ 18.0
NovaQuest | Private Placement
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares sold (in shares)				1,203,307
Net proceeds from sale of shares | $				$ 20.0
Tranche 1 | Common Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of securities exercisable by warrant (in shares)						49,800
Exercise price of warrants (in USD per share) | $ / shares						$ 15.06
Tranche 2 | Common Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of securities exercisable by warrant (in shares)										23,910
Exercise price of warrants (in USD per share) | $ / shares										$ 18.82
Subsequent Event | Cowen and Company, LLC | Private Placement
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares sold (in shares)			106,494
Net proceeds from sale of shares | $			$ 2.5