Income Taxes - Unrecognized Tax Benefit (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning balance	$ 3,246	$ 429	$ 491
Gross increase (decrease) for tax positions related to prior years	2,330	677	(310)
Gross increase for tax positions related to the current year	2,208	2,140	248
Ending balance	$ 7,784	$ 3,246	$ 429