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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, 2019
Mar. 31, 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 820 (176)
Net derivative contract liabilities - Gains (losses) included in other comprehensive income   (1)
Net derivative contract liabilities - Gains (losses) included in regulatory assets and liabilities (11) (9)
Net derivative contract liabilities - Acquisitions, dispositions and settlements (44) (26)
Net derivative contract liabilities - Ending Balance $ (1,578) $ (2,281)