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Fair value measurements - Significant unobservable inputs (Detail) - Significant Unobservable Inputs (Level 3) [Member] - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]      
Net derivative contract liabilities - Beginning Balance $ (857) $ (2,343) $ (2,069)
Net derivative contract liabilities - Gains (losses) included in earnings 603 1,972 (118)
Net derivative contract liabilities - Gains (losses) included in other comprehensive income   (1) 2
Net derivative contract liabilities - Gains (losses) included in regulatory assets and liabilities (17) (26) 3
Net derivative contract liabilities - Acquisitions 5 6 3
Net derivative contract liabilities - Dispositions and settlements (621) (465) (164)
Net derivative contract liabilities - Ending Balance (887) (857) (2,343)
Investments in Equity and Fixed Maturity Securities [Member]      
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]      
Beginning Balance 10,407 7 6
Gains (losses) included in earnings (1,426) 404  
Acquisitions   10,000 2
Dispositions and settlements (2) (4) (1)
Ending Balance $ 8,979 $ 10,407 $ 7