Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Pre-tax losses	$ 33,172	$ 19,466	$ 78,412	$ 53,625
Benefit from income taxes	$ 330	$ 66	$ 650	$ 264
Percentage of annual research and development credit for cash			65.00%
Income tax rate			21.00%		21.00%	35.00%

Amount of income (loss) from continuing operations, including income (loss) from equity method investments, before deduction of income tax expense (benefit), and income (loss) attributable to noncontrolling interest.

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