Marketable Securities - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]
Investments maturity period	1 year		1 year
Realized losses of available-for-sale securities	$ 0	$ 0
Investments other-than-temporarily impaired	$ 0		$ 0

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