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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, 2019
Sep. 30, 2018
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Net derivative contract liabilities - Beginning Balance $ (2,343) $ (2,069)
Net derivative contract liabilities - Gains (losses) included in earnings 1,528 446
Net derivative contract liabilities - Gains (losses) included in other comprehensive income (1)  
Net derivative contract liabilities - Gains (losses) included in regulatory assets and liabilities (40) (11)
Net derivative contract liabilities - Acquisitions, dispositions and settlements (282) (141)
Net derivative contract liabilities - Ending Balance (1,138) (1,775)
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 307 6
Gains (losses) included in earnings 499  
Acquisitions, dispositions and settlements 10,002 (1)
Ending Balance $ 10,808 $ 5