Marketable Securities - Additional Information (Detail) - USD ($)	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017
Investments Debt And Equity Securities [Abstract]
Realized losses	$ 0	$ 0
Investments other-than-temporarily impaired	$ 0	$ 0
Investments maturity period	1 year

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

No definition available.

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