Fair value measurements - Other information (Detail) - Significant Unobservable Inputs (Level 3) [Member] $ in Millions	9 Months Ended
Sep. 30, 2016 USD ($)
Other Investments [Member] \| Preferred Stock [Member] \| Discounted Cash Flow Method [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value, unobservable input, expected duration	6 years
Fair value, unobservable input, discount for transferability restrictions and subordination	1.59%
Fair value of asset	$ 11,615
Other Investments [Member] \| Common Stock Warrants [Member] \| Pricing Model, Warrant [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value, unobservable input, discount for transferability and hedging restrictions	7.00%
Fair value of asset	$ 5,502
Net Derivative Contract Liabilities [Member] \| Equity Index Put Options [Member] \| Pricing Model, Option [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value, unobservable input, volatility	21.00%
Fair value of liability	$ 3,973

The unobservable input used in a fair value measurement for the discount for transferability and hedging restrictions.

No definition available.

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- Definition

The unobservable input used in a fair value measurement for the discount for transferability restrictions and subordination.

+ References

No definition available.

+ Details

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- Definition

The unobservable input used in a fair value measurement for expected duration.

+ References

No definition available.

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- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ 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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- Definition

Measure of dispersion, in percentage terms (for instance, the standard deviation or variance), for a given stock price.

+ 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 (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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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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- Definition

Fair value of financial and nonfinancial obligations.

+ References

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