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Investments in Marketable Securities - Gross Realized Gains and Losses from Sale of Available for Sale Securities (Detail) - USD ($) $ in Thousands | 3 Months Ended
--- | ---
Mar. 31, 2021 | Mar. 31, 2020
Investments, Debt and Equity Securities [Abstract] |  |
Gross realized gains | $ 1 | $ 53
Gross realized losses | $ 0 | $ 0

X
- Definition This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. + References No definition available. + Details
Name: | us-gaap_AvailableForSaleSecuritiesGrossRealizedGains
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition This item represents the gross loss realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. + References No definition available. + Details
Name: | us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- References No definition available. + Details
Name: | us-gaap_InvestmentsDebtAndEquitySecuritiesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration