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Investments - Summary of Portion of Unrealized Gains and Losses Related to Equity Securities (Detail) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Investments Equity Securities [Abstract]        
Net gains and (losses) recognized during the period on equity securities $ (1,344) $ 1,686 $ (6,329) $ 1,659
Less: Net (gains) and losses recognized during the period on equity securities sold during the period (177) $ (1,686) (301) $ (1,659)
Unrealized gains and (losses) recognized during the reporting period on equity securities still held at the reporting period $ (1,521)   $ (6,630)