Income Taxes - Summary of Reconciliation of Beginning and Ending Amount of 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
Beginning balance	$ 2,460	$ 1,897	$ 2,615
Gross amount of increases in unrecognized tax benefits for tax positions taken in current year	547	538	499
Gross amount of decreases in unrecognized tax benefits for tax positions taken in prior year	(3)		(1,217)
Gross amount of increases in unrecognized tax benefits for tax positions taken in prior year		25
Ending balance	$ 3,004	$ 2,460	$ 1,897