Fair value measurements - Significant unobservable inputs (Detail) - Significant Unobservable Inputs (Level 3) [Member] - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016
Fair Value Measurements Significant Unobservable Inputs [Line Items]
Net derivative contract liabilities - Beginning Balance	$ (2,824)	$ (3,785)
Net derivative contract liabilities - Gains (losses) included in earnings	822	(221)
Net derivative contract liabilities - Gains (losses) included in other comprehensive income	(3)	(2)
Net derivative contract liabilities - Gains (losses) included in regulatory assets and liabilities	(5)	(12)
Net derivative contract liabilities - Acquisitions, dispositions and settlements	(78)	(81)
Net derivative contract liabilities - Transfers into/out of Level 3		195
Net derivative contract liabilities - Ending Balance	(2,088)	(3,906)
Fixed Maturities [Member]
Fair Value Measurements Significant Unobservable Inputs [Line Items]
Beginning Balance	64	100
Gains (losses) included in other comprehensive income	1	3
Acquisitions, dispositions and settlements	(58)	5
Transfers into/out of Level 3		(1)
Ending Balance	7	107
Equity Securities and Other Investments [Member]
Fair Value Measurements Significant Unobservable Inputs [Line Items]
Beginning Balance	17,257	21,403
Gains (losses) included in earnings		2,409
Gains (losses) included in other comprehensive income	1,156	(2,233)
Acquisitions, dispositions and settlements		(4,461)
Net derivative contract liabilities - Transfers into/out of Level 3	$ (18,413)
Ending Balance		$ 17,118

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 the income statement 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 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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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
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-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19279-110258

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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1a)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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-Publisher FASB
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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19279-110258

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Amount of transfers of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset) into (out of) level 3 of the fair value hierarchy.

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
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-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19279-110258

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