Income Taxes - Changes in Uncertain Tax Positions (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Beginning balance	$ 8,796	$ 7,558	$ 1,581
Increase in balances related to tax positions taken during the current period	1,738		3,196
Decrease in balances related to tax positions taken during the current period		(2)
Increase in balances related to tax positions taken during the prior period		1,240	2,781
Decrease in balances related to tax positions taken during the prior period	(9)
Decrease in balances due to change in United States statutory rate	(2,101)
Ending balance	$ 8,424	$ 8,796	$ 7,558