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Derivative contracts (Detail) - Not Designated as Hedging Instrument [Member] - Equity Index Put Options [Member] - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Derivative [Line Items]    
Derivative contract liabilities $ 1,065 $ 968
Notional value 10,991 14,385
Intrinsic value $ 727 $ 397
Weighted average remaining life (in years) 1 year 2 months 12 days 1 year 9 months 18 days