Income Taxes - Summary of Activity in Total Valuation Allowances (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of year	$ 7,608	$ 4,090	$ 2,376
Additions	6,852	3,518	1,882
Recoveries	0	0	(168)
Balance at end of year	$ 14,460	$ 7,608	$ 4,090