Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended
	Apr. 30, 2016	Sep. 30, 2018	Dec. 31, 2017
Product Liability Contingency [Line Items]
Accounts payable and accrued liabilities		$ 100
Operating Leases, Future Minimum Payments Due		17,500
Prepaid Expense, Current		257	$ 1,117
Other Commitment, Due in Next Twelve Months		1,000
Drug Supplier Project Plan [Member]
Product Liability Contingency [Line Items]
Prepaid Expense, Current		300
Other Commitment		1,700
Manufacturer of Prexigebersen [Member]
Product Liability Contingency [Line Items]
Other Commitment		1,000
Manufacture of Drug Substance [Member]
Product Liability Contingency [Line Items]
Other Commitment		600
Manufacturing Development [Member]
Product Liability Contingency [Line Items]
Other Commitment		100
Office Space
Product Liability Contingency [Line Items]
Operating Leases, Future Minimum Payments Due		$ 100
Lab Space
Product Liability Contingency [Line Items]
Operating Lease Monthly Rental Payments	$ 2,500