Marketable Securities - Additional Information (Detail) - USD ($)	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Investments Debt And Equity Securities [Abstract]
Realized losses	$ 0	$ 0
Investments other-than-temporarily impaired	$ 0	$ 0
Investments maturity period	1 year
Investments mature within 24 months	$ 5,000,000

This item represents the gross loss realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563

Name:	us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration