Leases - Summary of Operating Lease Costs Included in Research and Development and General and Administrative Expense, as well as Sublease Income (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Dec. 31, 2018
Leases [Abstract]
Operating lease costs	$ 2,131	$ 5,857
Short-term lease costs	1,117	3,378
Variable lease costs	698	2,111
Sublease income		(525)	$ (176)
Net lease cost	$ 3,946	$ 10,821