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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, 2019
Dec. 31, 2018
Dec. 31, 2017
Fair Value Measurements Significant Unobservable Inputs [Line Items]      
Net derivative contract liabilities - Beginning Balance $ (2,343) $ (2,069) $ (2,824)
Net derivative contract liabilities - Gains (losses) included in earnings 1,972 (118) 888
Net derivative contract liabilities - Gains (losses) included in other comprehensive income (1) 2 (3)
Net derivative contract liabilities - Gains (losses) included in regulatory assets and liabilities (26) 3 (1)
Net derivative contract liabilities - Acquisitions 6 3  
Net derivative contract liabilities - Dispositions and settlements (465) (164) (129)
Net derivative contract liabilities - Ending Balance (857) (2,343) (2,069)
Investments in Equity and Fixed Maturity Securities [Member]      
Fair Value Measurements Significant Unobservable Inputs [Line Items]      
Beginning Balance 7 6 17,321
Gains (losses) included in earnings 404    
Gains (losses) included in other comprehensive income     1,157
Acquisitions 10,000 2  
Dispositions and settlements (4) (1) (59)
Transfers into/out of Level 3     (18,413)
Ending Balance $ 10,407 $ 7 $ 6