Subsequent Events - Additional Information (Detail) $ in Millions			9 Months Ended
	Oct. 15, 2018 USD ($) shares	Oct. 08, 2018 USD ($) Installment	Sep. 30, 2018
Paragon Agreement [Member] | Paragon [Member]
Subsequent Event [Line Items]
Agreement expiration description			The term of the Paragon Agreement will continue until the earlier of: (i) the latest expiration date of any commercial supply agreement in effect, or (ii) December 31, 2025. The Company also has the ability to terminate the Paragon Agreement prior to expiration, subject to potential additional financial considerations.
Subsequent Event [Member] | Research and License Option Agreement [Member] | Nationwide Children's Hospital [Member]
Subsequent Event [Line Items]
Up-front cash payment under agreements		$ 1.0
Subsequent Event [Member] | Paragon Agreement [Member] | Paragon [Member]
Subsequent Event [Line Items]
One-time payment payable		$ 36.0
Number of installments | Installment		3
One-time payment payable expiration date		Mar. 31, 2019
Additional annual payments payable		$ 2.0
Agreement expiration date		Dec. 31, 2025
Subsequent Event [Member] | Equity Investment Agreement [Member] | Lysogne [Member]
Subsequent Event [Line Items]
Common stock purchased, shares | shares	950,606
Upfront license fee, option fee, equity investment and certain development activities reimbursement payable on execution of agreement	$ 28.0
Subsequent Event [Member] | Maximum [Member] | Research and License Option Agreement [Member] | Nationwide Children's Hospital [Member]
Subsequent Event [Line Items]
Development, regulatory and sales milestones payments to be paid		$ 29.0
Subsequent Event [Member] | Maximum [Member] | Equity Investment Agreement [Member] | Lysogne [Member]
Subsequent Event [Line Items]
Development, regulatory and sales milestones payments to be paid	$ 102.8