Income Taxes - Reconciliation of Amount of Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Beginning Balance	$ 10,116	$ 4,061	$ 332
Increases based on tax positions taken during a prior period		1,069	3,562
Gross (decrease) related to prior year tax positions	(7,776)
Increases based on tax positions taken during a current period	1,594	4,986	167
Ending Balance	$ 3,934	$ 10,116	$ 4,061