Income Taxes - Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefit, beginning of period	$ 940	$ 742
Gross increases — current year tax positions	327	198
Gross increases — prior year tax positions	73	0
Gross decreases — prior year tax positions	(205)	0
Unrecognized tax benefit, end of period	$ 1,135	$ 940