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Long-Term Investments - Rollforward of Credit (Gain) Loss Recognized in Earnings on Available-for-Sale Auction Rate Securities (Detail) - USD ($)
 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]    
Balance at the beginning of the period   $ 41,000
Reductions in the amount related to credit gain for which other-than-temporary impairment was not previously recognized   (3,000)
Balance at the end of the period   $ 38,000
Failed Auction Security [Member]    
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]    
Balance at the beginning of the period $ 37,000  
Reductions in the amount related to credit gain for which other-than-temporary impairment was not previously recognized (3,000)  
Balance at the end of the period $ 34,000