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Available-for-sale Securities and Fair Value on Financial Instruments - Additional Information (Detail) - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]          
Available-for-sale securities in unrealized net loss position         $ 0
Credit losses recognized $ 0 $ 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 0 $ 0 300,000 $ 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   $ 100,000