Derivative contracts - Narrative (Detail) - USD ($)	6 Months Ended
Jun. 30, 2018	Dec. 31, 2017
Finance and Financial Products [Member]
Derivative [Line Items]
Liabilities	$ 2,006,000,000	$ 2,172,000,000
Finance and Financial Products [Member] \| Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Collateral posting requirements under contracts with collateral provisions	0
Additional collateral posting requirements	$ 1,100,000,000
Finance and Financial Products [Member] \| Not Designated as Hedging Instrument [Member] \| Equity Index Put Options [Member]
Derivative [Line Items]
Weighted average remaining life of derivative contracts	2 years 6 months
Premiums received at contract inception dates	$ 4,100,000,000
Aggregate intrinsic value of equity index put option contracts	930,000,000	789,000,000
Liabilities	$ 2,006,000,000	2,172,000,000
Finance and Financial Products [Member] \| Not Designated as Hedging Instrument [Member] \| Equity Index Put Options [Member] \| Maximum [Member]
Derivative [Line Items]
Derivative inception month and year	2008-03
Derivative maturity month and year	2026-01
Finance and Financial Products [Member] \| Not Designated as Hedging Instrument [Member] \| Equity Index Put Options [Member] \| Minimum [Member]
Derivative [Line Items]
Derivative maturity month and year	2019-04
Other Assets [Member] \| Utilities and Energy [Member] \| Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Assets	$ 149,000,000	142,000,000
Accounts Payable, Accruals and Other Liabilities [Member] \| Utilities and Energy [Member] \| Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Liabilities	$ 93,000,000	$ 82,000,000

Month and year the the entity entered into the derivative contract, in YYYY-MM format.

No definition available.

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- Definition

The aggregate premiums received on derivative contracts at the contract inception dates.

+ References

No definition available.

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- Definition

Month and year the derivative contract matures, in YYYY-MM format.

+ References

No definition available.

+ Details

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- Definition

Represents the undiscounted liability assuming the contracts are settled on their future expiration dates based on the index values at the most recent balance sheet date.

+ References

No definition available.

+ Details

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- Definition

The aggregate fair value of additional assets that would be required to be posted as collateral for derivative instruments with credit-risk-related contingent features if the credit-risk-related contingent features were triggered at the end of the reporting period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4H
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624258-113959

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- Definition

The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4H
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-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624258-113959

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- Definition

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
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-Section 50
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- Definition

Average remaining period until maturity of the derivative contract, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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- Definition

Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
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-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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