Financial Instruments (Narrative) (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Financial Instruments [Line Items]
Realized loss or gains	$ 0	$ 0	$ 0
US Treasury and Government Short-term Debt Securities [Member]
Financial Instruments [Line Items]
Available for Sale Securities Maturity Period	3 years

Amount of realized gain (loss) on investment in debt and equity securities.

No definition available.

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