Collaboration, License and Manufacturing Agreements - Additional Information (Detail)	1 Months Ended		3 Months Ended			6 Months Ended
	Jun. 30, 2018 USD ($) Payment Month Quarter	May 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)
Warrant Agreement [Member] | Myonexus Therapeutics, Inc. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Up-front cash payment under agreements		$ 60,000,000
Development milestone payments evaluation period		2 years
Up-front payment					$ 60,000,000	$ 60,000,000
Development milestone payments					45,000,000
Warrant Agreement [Member] | Myonexus Therapeutics, Inc. [Member] | Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional development milestone payments to be paid		$ 45,000,000
Manufacturing Agreement [Member] | Brammer [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Advance upfront fee to be paid on execution of agreements	$ 20,000,000				20,000,000	$ 20,000,000
Number of non-refundable process equipment fee payments | Payment	2
Quarterly capacity access fee payment	$ 10,000,000
Portion of advance payment applied as credit to quarterly capacity access fees						one-tenth
Quarterly capacity access fees due and payable beginning date	Jul. 01, 2019
Quarterly capacity access fees due and payable ending date	Dec. 31, 2021
Net capacity access fee	$ 8,000,000
Manufacturing agreement term	6 years
Successive renewal term of manufacturing agreement	2 years
Maximum number of months prior to expiration of term for renewal | Month	24
Maximum additional quarters of remittance of capacity access fees required to terminate agreement | Quarter	8
Manufacturing Agreement [Member] | Brammer [Member] | Other Non-current Asset [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Advance upfront fee to be paid on execution of agreements	$ 20,000,000				20,000,000	$ 20,000,000
Manufacturing Agreement [Member] | Brammer [Member] | Other Asset [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Process equipment fee payments capitalized as other asset	$ 5,000,000				$ 5,000,000	$ 5,000,000
Manufacturing Agreement [Member] | Brammer [Member] | Scenario Forecast [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Non-refundable process equipment fee payments			$ 5,000,000	$ 5,000,000