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Fair value measurements - Significant unobservable inputs (Detail) - Significant Unobservable Inputs (Level 3) [Member] - USD ($)
$ in Millions
9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Net derivative contract liabilities - Beginning Balance $ (857) $ (2,343)
Net derivative contract liabilities - Gains (losses) included in earnings (72) 1,528
Net derivative contract liabilities - Gains (losses) included in regulatory assets and liabilities (36) (41)
Net derivative contract liabilities - Acquisitions, dispositions and settlements (398) (282)
Net derivative contract liabilities - Ending Balance (1,363) (1,138)
Investments in Fixed Maturity and Equity Securities [Member]    
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Beginning Balance 10,407 7
Gains (losses) included in earnings (2,235) 499
Acquisitions, dispositions and settlements (2) 10,002
Ending Balance $ 8,170 $ 10,508