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Marketable Securities - Additional Information (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Marketable Securities [Abstract]      
Fair value of securities owned $ 14,200 $ 32,100  
Realized and unrealized net gains (losses) on marketable securities 6,700 9,500 $ 24,000
Marketable securities sold at fair value 24,626 128,018 $ 38,256
Purchase of marketable securities $ 0 $ 0