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Available-for-sale Securities and Fair Value on Financial Instruments - Additional Information (Detail) - USD ($)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]      
Available-for-sale securities in unrealized net loss position   $ 0  
Credit losses recognized $ 0 0 $ 0
Gross realized gains from the sale or maturity of available-for-sale investments   0 0
Gross realized losses from the sale or maturity of available-for-sale investments 300,000 $ 0 $ 0
Maximum      
Schedule of Available-for-sale Securities [Line Items]      
Gross realized gains from the sale or maturity of available-for-sale investments $ 100,000