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Investments - Summary of Amounts Included in Net Realized (Losses) Gains and Other-Than-Temporary Impairments (Detail) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Investment [Line Items]    
Gross realized gains on securities $ 24,219 $ 11,054
Gross realized losses on securities (4,288) (1,388)
Foreign exchange (losses) gains 1,960 5,014
Net realized (losses) gains 17,233 4,862
Net other-than-temporary impairments (29) (299)
Gain (Loss) on Investments [Member]    
Investment [Line Items]    
Foreign exchange (losses) gains $ (2,698) $ (4,804)