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Financial Instruments Investments in Marketable Debt Securities - Accumulated Unrealized Losses on Debt Securities Classified as Available-for-Sale in Continuous Loss Position (Detail) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Schedule Of Available For Sale Securities [Line Items]    
Available-for-Sale Investments, Less than 12 months, Estimated Fair Value $ 42,762 $ 11,021
Available-for-Sale Investments, Less than 12 months, Gross Unrealized Losses (19) (3)
Available-for-Sale Investments, More than 12 months, Estimated Fair Value 0 4,245
Available-for-Sale Investments, More than 12 months, Gross Unrealized Losses 0 (8)
Corporate Debt Securities    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-Sale Investments, Less than 12 months, Estimated Fair Value 42,762 11,021
Available-for-Sale Investments, Less than 12 months, Gross Unrealized Losses (19) (3)
Available-for-Sale Investments, More than 12 months, Estimated Fair Value 0 1,497
Available-for-Sale Investments, More than 12 months, Gross Unrealized Losses $ 0 (4)
Government Sponsored Securities    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-Sale Investments, Less than 12 months, Estimated Fair Value   0
Available-for-Sale Investments, Less than 12 months, Gross Unrealized Losses   0
Available-for-Sale Investments, More than 12 months, Estimated Fair Value   2,748
Available-for-Sale Investments, More than 12 months, Gross Unrealized Losses   $ (4)