Employee Benefit Plan - Additional Information (Detail) - USD ($)	12 Months Ended	96 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Retirement Benefits [Abstract]
Company matched employee's contribution	50.00%	25.00%	25.00%
Employee's contribution	$ 4,000	$ 3,000	$ 2,000

Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.

No definition available.

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