Stock-based compensation - Range of exercise prices (Details) (Employee Stock Option, Long-term incentive and director compensation plans, USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Price, Minimum	$ 14.47
Range of Exercise Price, Maximum	$ 55.41
Number Outstanding (Shares)	57,647,446	64,930,540
Weighted Average Remaining Contractual Life (Years)	6 years 4 months 24 days
Options Outstanding Weighted Average Exercise Price per Share	$ 32.84	$ 28.98
Number Exercisable (Shares)	27,601,866
Options Exercisable Weighted Average Exercise Price per Share	$ 27.34
Range Of Exercise Prices $14.47 to $20.00
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Price, Minimum	$ 14.47
Range of Exercise Price, Maximum	$ 20.00
Number Outstanding (Shares)	4,061,577
Weighted Average Remaining Contractual Life (Years)	4 years
Options Outstanding Weighted Average Exercise Price per Share	$ 14.98
Number Exercisable (Shares)	4,061,577
Options Exercisable Weighted Average Exercise Price per Share	$ 14.98
Range Of Exercise Prices $20.01 to $30.00
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Price, Minimum	$ 20.01
Range of Exercise Price, Maximum	$ 30.00
Number Outstanding (Shares)	11,270,125
Weighted Average Remaining Contractual Life (Years)	3 years 8 months 12 days
Options Outstanding Weighted Average Exercise Price per Share	$ 25.47
Number Exercisable (Shares)	11,250,200
Options Exercisable Weighted Average Exercise Price per Share	$ 25.46
Range Of Exercise Prices $30.01 to $40.00
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Price, Minimum	$ 30.01
Range of Exercise Price, Maximum	$ 40.00
Number Outstanding (Shares)	28,910,636
Weighted Average Remaining Contractual Life (Years)	6 years 6 months
Options Outstanding Weighted Average Exercise Price per Share	$ 33.00
Number Exercisable (Shares)	12,288,339
Options Exercisable Weighted Average Exercise Price per Share	$ 33.14
Range Of Exercise Prices $40.01 to $50.00
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Price, Minimum	$ 40.01
Range of Exercise Price, Maximum	$ 50.00
Number Outstanding (Shares)	13,399,020
Weighted Average Remaining Contractual Life (Years)	9 years 1 month 6 days
Options Outstanding Weighted Average Exercise Price per Share	$ 44.10
Number Exercisable (Shares)	1,750
Options Exercisable Weighted Average Exercise Price per Share	$ 42.66
Range Of Exercise Prices $50.01 to $55.41
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Price, Minimum	$ 50.01
Range of Exercise Price, Maximum	$ 55.41
Number Outstanding (Shares)	6,088
Weighted Average Remaining Contractual Life (Years)	9 years 10 months 24 days
Options Outstanding Weighted Average Exercise Price per Share	$ 55.41
Number Exercisable (Shares)	0

Number of options outstanding, including both vested and non-vested options.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

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

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(1)(i)-(ii)

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

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- Definition

Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(1)(i)

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

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- Definition

Weighted average exercise price as of the balance sheet date for those equity-based payment arrangements exercisable and outstanding.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(1)(iii)

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

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- Definition

The floor of a customized range of exercise prices for purposes of disclosing shares potentially issuable under outstanding stock option awards on all stock option plans and other required information pertaining to awards in the customized range.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (g)

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

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Balance Type:	na
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- Definition

The number of shares reserved for issuance pertaining to the outstanding exercisable stock options as of the balance sheet date in the customized range of exercise prices for which the market and performance vesting condition has been satisfied.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(iii)

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

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- Definition

Weighted average remaining contractual term of outstanding stock options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (e)(1)

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

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- Definition

The ceiling of a customized range of exercise prices for purposes of disclosing shares potentially issuable under outstanding stock option awards on all stock option plans and other required information pertaining to awards in the customized range.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (g)

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

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