Fair Value of Financial Instruments - Additional Information (Detail)	6 Months Ended	12 Months Ended
Jun. 30, 2018	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Fair value transfers between Level 1 and Level 2	no	no

Description of the transfers of assets measured on a recurring basis out of Level 1 and into Level 2 of the fair value hierarchy.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bb)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

Name:	us-gaap_FairValueAssetsLevel1ToLevel2TransfersDescription
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration