Reconciliations of Total Amount of Gross Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Schedule of Unrecognized Tax Benefits [Line Items]
Gross unrecognized tax benefits at beginning of year	$ 8,293	$ 7,621
Increase in tax positions for prior years	298	14
Increases in tax positions for current year	4,724	1,944
Lapse in statute of limitations	(754)	(1,286)
Gross unrecognized tax benefits at end of year	$ 12,561	$ 8,293