Segment Reporting - Additional Information (Detail)	12 Months Ended
Dec. 31, 2017 subsidiary
Public Subsidiaries
Segment Reporting Information [Line Items]
Number of subsidiaries	2

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

Name:	us-gaap_SegmentReportingInformationLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration