Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
Dec. 31, 2017 Segment
Summary Of Significant Accounting Policies [Line Items]
Maximum period of investments with original maturities from date of acquisition to be cash equivalents	3 months
Number of business segments	1
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives	3 years
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives	7 years

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