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Fair value measurements - Significant unobservable inputs (Detail) - Significant Unobservable Inputs (Level 3) [Member] - USD ($)
$ in Millions
6 Months Ended
Jun. 30, 2020
Jun. 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 (265) 1,134
Net derivative contract liabilities - Gains (losses) included in other comprehensive income   (1)
Net derivative contract liabilities - Gains (losses) included in regulatory assets and liabilities (56) (23)
Net derivative contract liabilities - Acquisitions, dispositions and settlements (265) (128)
Net derivative contract liabilities - Ending Balance (1,443) (1,361)
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 (2,469)  
Acquisitions, dispositions and settlements   (2)
Ending Balance $ 7,938 $ 5