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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 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 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 $ 573 $ (2,473) $ 3,339 $ (9,103)