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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, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Investments Equity Securities [Abstract]        
Net gains and (losses) recognized during the period on equity securities $ (2,060) $ 520 $ (8,389) $ 2,179
Less: Net (gains) and losses recognized during the period on equity securities sold during the period (413) $ (520) (714) $ (2,179)
Unrealized gains and (losses) recognized during the reporting period on equity securities still held at the reporting period $ (2,473)   $ (9,103)