Commitments and Contingencies - Additional Information (Detail) $ in Millions	1 Months Ended	12 Months Ended
	May 31, 2016 ft²	Dec. 31, 2017 USD ($) ft²	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Apr. 03, 2017 ft²
Operating Leased Assets [Line Items]
Amended lease termination date		Jun. 30, 2019
Aggregate space that the entity will lease under the amended lease agreement | ft²		112,300
Aggregate landlord reimbursable tenant improvements received under the existing lease and the lease amendment | $			$ 9.5
Sublease Agreement [Member]
Operating Leased Assets [Line Items]
Area of office and lab | ft²	8,143
Lease expiration date	Jun. 30, 2019
Operating leases, future minimum rental payments receivable | $		$ 0.4
Total rent expense | $		$ 3.6	$ 5.0	$ 4.3
Sublease Agreement [Member] | Ipsen [Member]
Operating Leased Assets [Line Items]
Area of sublease property | ft²					70,237