Income Taxes - Details of Beginning and Ending Amount of Gross Unrecognized Tax Benefits (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]
Balance at beginning of year	$ 6,232	$ 6,285	$ 1,744
Gross increases for tax positions of prior years	0	0	141
Gross decreases for federal tax rate change for tax positions of prior years	(1,670)	(22)	(15)
Gross increases for tax positions of current year	110	111	4,415
Lapse of statute of limitations	0	(142)	0
Balance at end of year	$ 4,672	$ 6,232	$ 6,285