Income Taxes - Activity Related to Our 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	$ 1,960	$ 1,774
Gross increases, tax positions in current period	98	185
Gross increases, tax positions in prior period	0	1
Gross decreases, tax positions in prior period	(3)	0
Unrecognized tax benefit, end of period	$ 2,055	$ 1,960