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Marketable Securities - Additional Information (Detail) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Marketable Securities [Abstract]      
Fair value of securities owned $ 30,000   $ 32,100
Marketable securities 29,989   $ 32,064
Realized and unrealized net gains (losses) on marketable securities 2,500 $ 9,300  
Marketable securities sold at fair value 4,553 72,366  
Purchase of marketable securities $ 0 $ 0