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Investments - Summary of Details on Realized and Unrealized Gains and Losses Related to Equity Securities (Detail) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Net Investment Income [Line Items]        
Unrealized gains and (losses) recognized during the reported period on equity securities still held at the end of the reported period $ 3,871 $ 3,759 $ (4,153) $ 21,791
Equity Securities [Member]        
Net Investment Income [Line Items]        
Unrealized gains and (losses) recognized during the reported period on equity securities still held at the end of the reported period $ 3,871 $ 880 $ (4,154) $ 2,766