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Investments - Realized and Unrealized Gains and Losses related to Marketable Equity Securities (Detail) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Investments Debt And Equity Securities [Abstract]    
Realized gains (losses) on equity securities sold $ 24 $ (2,436)
Unrealized gains (losses) on equity securities held 901 (8,464)
Total gain (loss) recognized, net $ 925 $ (10,900)