Fair Value Measurements - Additional Information (Details) - USD ($)	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Transfers between Level I, Level II or Level III	$ 0	$ 0	$ 0	$ 0
U.S. Treasury Securities
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Transfers between Level 1 and Level 2	$ 0	$ 0	$ 0	$ 0

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

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_FairValueAssetsLevel1ToLevel2TransfersAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant