Income Taxes - Summary of Reconciliation of Unrecognized Tax Benefit Activity (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Beginning unrecognized tax benefits	$ 1,532,000	$ 1,408,000
Decreases related to prior year tax positions	(1,001,000)	(53,000)
Increases related to current year tax positions	179,000	177,000
Ending unrecognized tax benefits	$ 710,000	$ 1,532,000