Nature of the Business - Additional Information (Detail)		1 Months Ended
	Oct. 16, 2017 USD ($)	Sep. 30, 2017 USD ($) Program	Apr. 03, 2017 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of development programs active | Program		2
Ipsen [Member] | Asset Sale Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash payment received			$ 575,000,000
Working capital adjustments received		$ 800,000
Ipsen [Member] | Asset Sale Agreement [Member] | Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional payments receivable on achievement of certain milestone events			450,000,000
Baxalta [Member] | License and Collaboration Agreements [Member] | Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Net milestone payments expected to be received			$ 33,000,000
Subsequent Event [Member] | Ipsen [Member] | Asset Sale Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Working capital adjustments received	$ 4,900,000