Stock-based compensation (Details Textual) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total intrinsic value of stock options exercised during the year	$ 47	$ 15	$ 57
Expected dividend yield	2.00%	0.00%	0.00%
Weighted-average risk-free interest rate	2.50%	3.20%	2.10%
Performance Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
The number of shares by which the pool of available shares will be reduced for other types of awards granted	1.9	1.9	1.9
Period over which the grants of equity instruments vest	3 years	3 years	3 years
Expected dividend yield	0.10%	0.00%	0.00%
Weighted-average risk-free interest rate	1.20%	1.30%	1.20%
Weighted average grant date fair value, granted	$ 49.50	$ 62.06	$ 48.22
Total fair value of units that vested during the year	25	34	29
Total unrecognized compensation cost related to nonvested awards	90
Weighted average number of years over which compensation cost related to nonvested awards is expected to be recognized	1.1
Units outstanding	4,100,000	2,700,000
Weighted average grant date fair value of units, outstanding	$ 51.92	$ 49.49
Units granted	2,500,000
Units, vested	400,000
Weighted average grant date fair value, vested	$ 48.22
Units, forfeited	200,000
Weighted average grant date fair value, forfeited	$ 52.70
Performance unit liability awards paid			30
Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
The number of shares by which the pool of available shares will be reduced for other types of awards granted	1.9	1.9	1.9
Weighted average grant date fair value, granted	$ 51.83	$ 58.19	$ 51.24
Total fair value of units that vested during the year	176	184	139
Units outstanding	9,000,000	9,300,000
Units granted	4,000,000
Units, vested	3,400,000
Weighted average grant date fair value, vested	$ 52.06
Units, forfeited	900,000
Weighted average grant date fair value, forfeited	$ 52.77
Stock Option and Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized compensation cost related to nonvested awards	$ 407
Weighted average number of years over which compensation cost related to nonvested awards is expected to be recognized	1.7
Granted on and after April 25, 2011 [Member] \| Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield	1.90%
Weighted-average risk-free interest rate	1.00%
Granted on and after April 26, 2010 [Member] \| Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expiration period of stock options from date of grant	P10Y	P10Y
Granted prior to April 26, 2010 [Member] \| Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expiration period of stock options from date of grant		P7Y	P7Y
Granted prior to April 25, 2011 [Member] \| Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Period over which the grants of equity instruments vest	4 years	4 years	4 years
Granted prior to April 25, 2011 [Member] \| Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Period over which the grants of equity instruments vest	4 years	4 years	4 years

Amount by which the pool of available shares will be reduced for other types of awards granted.

No definition available.

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- Definition

Weighted average grant date fair value of units outstanding.

+ References

No definition available.

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- Definition

As of the balance sheet date, the aggregate unrecognized cost of equity-based awards made to employees under equity-based compensation awards that have yet to vest.

+ References

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- Definition

The weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, using a decimal to express in number of years.

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- Definition

The date when the equity-based award expires as specified in the award agreement, which may be presented in a variety of ways (for example, year, month and year, day, month and year, quarter of a year).

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- Definition

Description of the estimated period of time over which an employee is required to provide service in exchange for the equity-based payment award, which often is the vesting period. This period may be explicit or implicit based on the terms of the award, and may be presented in a variety of ways (for example, year, month and year, day, month and year, quarter of a year).

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- Definition

The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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- Definition

The weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect during the reporting period as a result of the occurrence of a terminating event specified in the contractual agreement of the plan.

+ References

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- Definition

The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

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- Definition

The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph b(2)(c)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph c(1)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(2)(iii)(1)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph b(2)(a)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph b(2)(b)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(2)(i)-(ii)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

The liabilities paid during the accounting period for equity-based compensation.

+ References

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- Definition

The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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- Definition

The total fair value of equity-based awards for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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- Definition

The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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- Definition

The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

+ References

+ Details

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- Definition

The risk-free interest rate assumption that is used in valuing an option on its own shares.

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- Definition

The total accumulated difference between fair values of underlying shares on dates of exercise and exercise price on options which were exercised (or share units converted) into shares during the reporting period under the plan.

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