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Available-for-sale Securities and Fair Value on Financial Instruments - Additional Information (Detail) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]          
Available-for-sale securities in unrealized net loss position $ 0   $ 0   $ 283,000
Other-than-temporary impairment losses 0 $ 0 0 $ 0  
Gross realized gains from the sale or maturity of available-for-sale investments 0 0 0 0  
Gross realized losses from the sale or maturity of available-for-sale investments $ 0 $ 0 $ 0 $ 0