Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
Dec. 31, 2018 Segment shares	Dec. 31, 2017 shares
Accounting Policies [Abstract]
Number of operating segment \| Segment	1
Shares of common stock subject to repurchase excluded from the weighted-average number of common stock outstanding \| shares	18,791	45,000

Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues.

No definition available.

Name:	us-gaap_NumberOfOperatingSegments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration