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Fair value measurements - Significant unobservable inputs (Detail) - Significant Unobservable Inputs (Level 3) [Member] - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2020
Mar. 31, 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 (1,324) 820
Net derivative contract liabilities - Gains (losses) included in regulatory assets and liabilities (40) (11)
Net derivative contract liabilities - Acquisitions, dispositions and settlements (43) (44)
Net derivative contract liabilities - Ending Balance (2,264) (1,578)
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 (4,861)  
Acquisitions, dispositions and settlements   (1)
Ending Balance $ 5,546 $ 6