Collaboration and License Agreements - Additional Information (Detail) £ in Millions	1 Months Ended						3 Months Ended	9 Months Ended	12 Months Ended
	Nov. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2016 GBP (£)	May 31, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Oct. 31, 2017 USD ($)
Aviragen Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential milestone payment upon achievement of specified clinical, regulatory and commercial milestones						$ 12,000,000
Cantab Related Agreements [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential milestone payment upon achievement of specified clinical and regulatory milestones			$ 5,800,000
Potential milestone payment upon achievement of specified commercial milestone				£ 5.0				$ 6,500,000	$ 6,700,000
Contract termination period			10 years	10 years
Vertex License Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract termination period								10 years
Nonrefundable upfront payments					$ 500,000
Potential milestone payment upon achievement of specified clinical, regulatory and commercial milestones					$ 81,100,000
Contract termination period if no material development or commercialization occurs								1 year
Meiji License Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential milestone payment upon achievement of specified clinical and regulatory milestones		$ 3,000,000
Nonrefundable upfront payments		600,000
Potential milestone payments upon achievement of specified condition										$ 1,000,000
Meiji License Agreement [Member] | Scenario Forecast [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
License agreement fixed assets related payments							$ 1,600,000
Meiji License Agreement [Member] | Maximum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Sublicense fee payable to counter party		7,500,000
Northern License Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential milestone payment upon achievement of specified clinical, regulatory and other milestones		7,000,000
Potential milestone payment upon closing of initial public offering		2,500,000
Northern Exchange Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential milestone payment upon closing of initial public offering		$ 100,000
Northern License And Exchange Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential milestone payment upon closing of initial public offering	$ 2,600,000