Accounts Payable and Accrued Liabilities - Summary of Accounts Payable and Accrued Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Accounts payable	$ 16	$ 2,580
Accrued research and development expenses	7,898	2,887
Accrued professional fees	1,318	270
Deferred rent	417	498
Other	1,386	26
Total accounts payable and accrued liabilities	$ 11,035	$ 6,261