Fair value measurements - Other information (Detail) - Significant Unobservable Inputs (Level 3) [Member] $ in Millions	3 Months Ended
Mar. 31, 2017 USD ($)
Preferred Stock [Member] \| Discounted Cash Flow [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other investments, Fair Value	$ 7,730
Common Stock Warrants [Member] \| Warrant Pricing Model [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other investments, Fair Value	10,682
Equity Index Put Options [Member] \| Option Pricing Model [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative liabilities, Fair Value	$ 2,430
Other Investments [Member] \| Preferred Stock [Member] \| Discounted Cash Flow [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value, unobservable input, expected duration	7 years
Fair value, unobservable input, discount for transferability restrictions and subordination	1.45%
Other Investments [Member] \| Common Stock Warrants [Member] \| Warrant Pricing Model [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value, unobservable input, discount for transferability and hedging restrictions	4.00%
Net Derivative Liabilities [Member] \| Equity Index Put Options [Member] \| Option Pricing Model [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value, unobservable input, volatility	19.00%

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.

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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, 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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=51825145&loc=SL20226008-175313

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13495-108611

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
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-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13433-108611

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958

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-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

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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

Amount of investments classified as other.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.12)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1(f,g))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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