Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended		12 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Contingencies And Commitments [Line Items]
Research and development expense	[1]	$ 39,820	$ 17,845	$ 84,972	$ 49,770
Cost sharing expensed relating to patent maintenance, defense and prosecution, incurred		500	100	1,600	1,100
Cost sharing expensed relating to patent maintenance, defense and prosecution, accrued				1,400					$ 400
Research Institutions [Member] | Scenario Forecast [Member]
Contingencies And Commitments [Line Items]
Research and development expense						$ 800	$ 800	$ 800
Research Agreements [Member]
Contingencies And Commitments [Line Items]
Research and development expense		300	$ 400	1,400	$ 2,100
Research Agreements [Member] | Scenario Forecast [Member]
Contingencies And Commitments [Line Items]
Research and development expense						$ 1,000	$ 1,000	$ 1,000
Manufacturing Agreements [Member] | Prepaid Expenses [Member]
Contingencies And Commitments [Line Items]
Upfront payment recorded as prepaid expense		$ 2,400		$ 2,400

[1] Including the following research and development expense with a related party, see Notes 6 & 10: