Financial Instruments Investments in Marketable Debt Securities - Schedule of Realized Gains and Losses on Sales of Available-for-Sale Debt Securities (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Realized Gain (Loss) [Abstract]
Available-for-sale Securities, Gross Realized Gains	$ 4	$ 4
Available-for-sale Securities, Gross Realized Losses	(13)	(8)
Available for Sale Securities, Net Realized Gains (Losses)	$ (9)	$ (4)

This item represents the difference between the gross realized gains and losses realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. Additionally, this item would include any losses recognized for other than temporary impairments (OTTI) of the subject investments in debt and equity securities.

No definition available.

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