Financial Instruments - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
Mar. 31, 2019	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Unrealized gain or loss on available-for-sale securities	$ 0	$ 0

Amount of unrealized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

No definition available.

Name:	us-gaap_DebtSecuritiesAvailableForSaleUnrealizedGainLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration