Prepaid Drug Product for Testing - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Inventory Disclosure [Line Items]
Prepaid drug product for testing		$ 1,100
Payments to Suppliers	$ 100
Prepaid Expense, Current	$ 1,256	1,117
Drug Supplier Project Plan [Member]
Inventory Disclosure [Line Items]
Prepaid Expense, Current		$ 1,300