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Investments - Realized and Unrealized Gains and Losses for Marketable Equity Securities (Detail) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Investments Debt And Equity Securities [Abstract]    
Realized gains (losses) on equity securities sold $ (14) $ 398
Unrealized gains (losses) on equity securities held 5,918 (422)
Total gain (loss) recognized, net $ 5,904 $ (24)