Shareholders' Equity (Deficit)	12 Months Ended
Mar. 31, 2019
Equity [Abstract]
Shareholders' Equity (Deficit)	Shareholders’ Equity (Deficit) (A) Overview: The Company’s Memorandum of Association, filed on February 2, 2016 in Bermuda, authorized the creation of one class of shares. As of March 31, 2019, the Company had 564,111,242 shares authorized with a par value of $0.000017727 per share. (B) Initial Public Offering and Reverse Stock Split: On October 18, 2016, the Company’s board of directors approved a 1-for-1.7727 reverse stock split of the Company’s outstanding common shares. The reverse stock split became effective on October 18, 2016. These consolidated financial statements give retroactive effect to the reverse stock split for all periods presented. On November 1, 2016, the Company completed its IPO of common shares. The Company sold 14,500,000 common shares at a price of $15.00 per common share, for gross proceeds of $217.5 million. The Company received net proceeds of $200.0 million, after deducting $15.2 million in underwriting discounts and commissions and $2.3 million in offering costs paid by the Company. (C) Underwritten Public Equity Offering of Common Shares: In July and August 2018, the Company completed an underwritten public equity offering of 3,533,399 of its common shares (including 200,065 common shares issued and sold upon the partial exercise of the underwriters’ option to purchase additional shares) at a public offering price of $22.50 per common share. After deducting the underwriting discounts and commissions and offering costs paid by the Company, the net proceeds to the Company in connection with the underwritten public equity offering, including from the partial option exercise, were approximately $74.4 million. (D) Private Placement with RSL: In April 2018, the Company entered into a share purchase agreement, or the Purchase Agreement, with RSL, its controlling shareholder, pursuant to which the Company sold to RSL 1,110,015 of its common shares at a purchase price of $20.27 per common share, for gross proceeds of $22.5 million, in a private placement, or the Private Placement. (E) At-the-Market Equity Offering Program: In April 2018, the Company entered into a sales agreement, or the Sales Agreement, with Cowen and Company, LLC, or Cowen, to sell its common shares having an aggregate offering price of up to $100.0 million from time to time through an “at-the-market” equity offering program under which Cowen acts as the Company’s agent. During the year ended March 31, 2019, the Company issued and sold 3,970,129 of its common shares under the Sales Agreement. The common shares were sold at a weighted-average price of $21.91 per common share for aggregate net proceeds to the Company of approximately $84.1 million, after deducting underwriting commissions and offering costs paid by the Company. As of March 31, 2019, the Company had approximately $13.0 million of capacity available to it under its “at-the-market” equity offering program. In April 2019, the Company issued and sold an additional 106,494 common shares for aggregate net proceeds of $2.5 million pursuant to this program. (F) Issuance of Equity Instruments to NovaQuest and Hercules: In October 2017, the Company issued and sold 138,361 common shares to NovaQuest for $2.0 million in accordance with the terms of the NovaQuest Securities Purchase Agreement. In December 2018, the Company issued and sold 1,082,977 common shares to NovaQuest for $18.0 million in accordance with the NovaQuest Securities Purchase Agreement and issued and sold 1,203,307 common shares to NovaQuest for $20.0 million in accordance with the NovaQuest Equity Purchase Agreement. In October 2017, the Company issued a Warrant to Hercules exercisable for 49,800 of its common shares at an exercise price of $15.06 per common share and in March 2018, the Company issued a Warrant to Hercules exercisable for an aggregate of 23,910 of its common shares at an exercise price of $18.82 per common share. Additional information is included in Note 5, “Financing Arrangements.” (G) Takeda Warrant Liability: During the year ended March 31, 2017, the Company issued 2,339,192 common shares to Takeda upon the automatic exercise of the Takeda warrant, which was due to the issuance of 153,846 common shares initiated by the grant of a restricted share award for 1,128,222 common shares, issuance of 208,077 common shares initiated by the grant of options to purchase 1,525,857 common shares and the issuance of an additional 1,977,269 common shares to Takeda upon the closing of the Company’s IPO, based upon the sale and issuance of 14,500,000 common shares to investors in the IPO. During the year ended March 31, 2018, the Company issued 4,432 common shares to Takeda upon the automatic exercise of the Takeda warrant, which was initiated by the grant of options to purchase 32,500 common shares in April 2017. The warrant expired on April 30, 2017.

Shareholders’ Equity (Deficit)

(A) Overview:

The Company’s Memorandum of Association, filed on February 2, 2016 in Bermuda, authorized the creation of one class of shares. As of March 31, 2019, the Company had 564,111,242 shares authorized with a par value of $0.000017727 per share.

(B) Initial Public Offering and Reverse Stock Split:

On October 18, 2016, the Company’s board of directors approved a 1-for-1.7727 reverse stock split of the Company’s outstanding common shares. The reverse stock split became effective on October 18, 2016. These consolidated financial statements give retroactive effect to the reverse stock split for all periods presented.

On November 1, 2016, the Company completed its IPO of common shares. The Company sold 14,500,000 common shares at a price of $15.00 per common share, for gross proceeds of $217.5 million. The Company received net proceeds of $200.0 million, after deducting $15.2 million in underwriting discounts and commissions and $2.3 million in offering costs paid by the Company.

In July and August 2018, the Company completed an underwritten public equity offering of 3,533,399 of its common shares (including 200,065 common shares issued and sold upon the partial exercise of the underwriters’ option to purchase additional shares) at a public offering price of $22.50 per common share. After deducting the underwriting discounts and commissions and offering costs paid by the Company, the net proceeds to the Company in connection with the underwritten public equity offering, including from the partial option exercise, were approximately $74.4 million.

(D) Private Placement with RSL:

In April 2018, the Company entered into a share purchase agreement, or the Purchase Agreement, with RSL, its controlling shareholder, pursuant to which the Company sold to RSL 1,110,015 of its common shares at a purchase price of $20.27 per common share, for gross proceeds of $22.5 million, in a private placement, or the Private Placement.

(E) At-the-Market Equity Offering Program:

In April 2018, the Company entered into a sales agreement, or the Sales Agreement, with Cowen and Company, LLC, or Cowen, to sell its common shares having an aggregate offering price of up to $100.0 million from time to time through an “at-the-market” equity offering program under which Cowen acts as the Company’s agent. During the year ended March 31, 2019, the Company issued and sold 3,970,129 of its common shares under the Sales Agreement. The common shares were sold at a weighted-average price of $21.91 per common share for aggregate net proceeds to the Company of approximately $84.1 million, after deducting underwriting commissions and offering costs paid by the Company. As of March 31, 2019, the Company had approximately $13.0 million of capacity available to it under its “at-the-market” equity offering program. In April 2019, the Company issued and sold an additional 106,494 common shares for aggregate net proceeds of $2.5 million pursuant to this program.

(F) Issuance of Equity Instruments to NovaQuest and Hercules:

In October 2017, the Company issued and sold 138,361 common shares to NovaQuest for $2.0 million in accordance with the terms of the NovaQuest Securities Purchase Agreement. In December 2018, the Company issued and sold 1,082,977 common shares to NovaQuest for $18.0 million in accordance with the NovaQuest Securities Purchase Agreement and issued and sold 1,203,307 common shares to NovaQuest for $20.0 million in accordance with the NovaQuest Equity Purchase Agreement. In October 2017, the Company issued a Warrant to Hercules exercisable for 49,800 of its common shares at an exercise price of $15.06 per common share and in March 2018, the Company issued a Warrant to Hercules exercisable for an aggregate of 23,910 of its common shares at an exercise price of $18.82 per common share. Additional information is included in Note 5, “Financing Arrangements.”

(G) Takeda Warrant Liability:

During the year ended March 31, 2017, the Company issued 2,339,192 common shares to Takeda upon the automatic exercise of the Takeda warrant, which was due to the issuance of 153,846 common shares initiated by the grant of a restricted share award for 1,128,222 common shares, issuance of 208,077 common shares initiated by the grant of options to purchase 1,525,857 common shares and the issuance of an additional 1,977,269 common shares to Takeda upon the closing of the Company’s IPO, based upon the sale and issuance of 14,500,000 common shares to investors in the IPO. During the year ended March 31, 2018, the Company issued 4,432 common shares to Takeda upon the automatic exercise of the Takeda warrant, which was initiated by the grant of options to purchase 32,500 common shares in April 2017. The warrant expired on April 30, 2017.