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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, 2019
Jun. 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 1,134 256
Net derivative contract liabilities - Gains (losses) included in other comprehensive income (1)  
Net derivative contract liabilities - Gains (losses) included in regulatory assets and liabilities (23) (14)
Net derivative contract liabilities - Acquisitions, dispositions and settlements (128) (79)
Net derivative contract liabilities - Ending Balance $ (1,361) $ (1,906)