Fair value measurements - Significant unobservable inputs (Detail) - Significant Unobservable Inputs (Level 3) [Member] - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017
Fair Value Measurements Significant Unobservable Inputs [Line Items]
Net derivative contract liabilities - Beginning Balance	$ (2,069)	$ (2,824)
Net derivative contract liabilities - Gains (losses) included in earnings	446	822
Net derivative contract liabilities - Gains (losses) included in other comprehensive income		(3)
Net derivative contract liabilities - Gains (losses) included in regulatory assets and liabilities	(11)	(5)
Net derivative contract liabilities - Acquisitions, dispositions and settlements	(141)	(78)
Net derivative contract liabilities - Ending Balance	(1,775)	(2,088)
Investments in Equity and Fixed Maturity Securities [Member]
Fair Value Measurements Significant Unobservable Inputs [Line Items]
Beginning Balance	6	17,321
Gains (losses) included in other comprehensive income		1,157
Acquisitions, dispositions and settlements	(1)	(58)
Transfers into/out of Level 3		(18,413)
Ending Balance	$ 5	$ 7

Fair Value Measurements Significant Unobservable Inputs [Line Items]

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Amount of gain (loss) recognized in regulatory assets and liabilities of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

No definition available.

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Amount of gain (loss) recognized in other comprehensive income (loss) for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of purchases, (sales), issuances and (settlements) of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of transfer of financial instrument classified as an asset into (out of) level 3 of the fair value hierarchy.

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No definition available.

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Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of gain (loss) recognized in the income statement of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of gain (loss) recognized in other comprehensive income (loss) of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of purchases, (sales), issuances and (settlements) of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Fair value of financial instrument classified as derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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