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Marketable Securities (Detail) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Dec. 31, 2020
Marketable Securities [Abstract]          
Fair value of securities owned $ 400   $ 400   $ 300
Realized and unrealized net gains (loss) on marketable securities $ 21 $ 33 10 $ 300  
Marketable securities sold at fair value during period     0 14,200  
Purchase of marketable securities     $ 0 $ 0