Employee 401(k) Savings Plan - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Postemployment Benefits [Abstract]
Defined Contribution Plan, Plan Name	401(k)
Maximum employee plan contribution percentage of compensation	50.00%
Company plan contributions	$ 590	$ 519	$ 392

Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan.

No definition available.

Name:	us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration