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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, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Investments, Debt and Equity Securities [Abstract]        
Unrealized gains and (losses) recognized during the reporting period on equity securities still held at the reporting period $ 880   $ 2,766  
Unrealized gains and (losses) recognized during the reporting period on equity securities still held at the reporting period $ 3,759 $ (1,521) $ 21,791 $ (6,630)