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Investments - Summary of Amounts Included in Net Realized (Losses) Gains and Other-Than-Temporary Impairments (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Investment [Line Items]      
Gross realized gains on securities $ 38 $ 64 $ 111
Gross realized losses on securities (12) (5) (7)
Foreign exchange (losses) gains (1) 12 (7)
Debt Securities, Available-for-sale, Allowance for Credit Loss, Not to Sell before Recovery, Credit Loss, Previously Recorded, Expense (Reversal) 0 0 0
Debt Securities, Available-for-sale, Allowance for Credit Loss, Sell before Recovery 0 0 (3)
Net realized (losses) gains 22 81 108
Gain (Loss) on Investments [Member]      
Investment [Line Items]      
Foreign exchange (losses) gains $ (4) $ 22 $ 7