Earnings per share (Details) (USD $) In Millions, except Per Share data	3 Months Ended	9 Months Ended
Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Numerator:
Net income	$ 165	$ 367	$ 64	$ 632
Denominator:
Weighted average common shares outstanding (in shares)	512.0	540.9	520.4	540.6
Effect of dilutive potential common shares:
Stock options (in shares)	1.6	1.9	2.0	2.0
Restricted stock units (non-participating) (in shares)	0.6	0.2	0.5	0.1
Weighted average common and dilutive potential common shares outstanding (in shares)	514.2	543.0	522.9	542.7
Earnings per share - Basic (in dollars per share)	$ 0.32	$ 0.68	$ 0.12	$ 1.17
Earnings per share - Diluted (in dollars per share)	$ 0.32	$ 0.68	$ 0.12	$ 1.16
Other Earnings Per Share Disclosures
Antidilutive stock options, exercise price exceeds market price (in shares)	27.6	27.6	27.6	26.6
Antidilutive stock options, exercise price exceeds market price, exercise price range, low end of range (in dollars per share)	$ 24.70	$ 27.36	$ 25.91	$ 27.36
Antidilutive stock options, exercise price exceeds market price, exercise price range, high end of range (in dollars per share)	$ 62.84	$ 62.84	$ 62.84	$ 64.53

The high end of the range of specified exercise prices for stock options with exercise prices exceeding the average market price of common shares during the period or for which the unrecognized compensation cost would have an anti-dilutive effect.

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Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

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