Postretirement benefit plans - Weighted average assumptions used (Details)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
U.S. defined benefit plan
Weighted average assumptions used to determine benefit obligations [Abstract]
Discount rate (in percents)	4.23%	5.11%
Long-term pay progression (in percents)	3.30%	3.50%
Weighted average assumptions used to determine net periodic benefit cost [Abstract]
Discount rate (in percents)	5.11%	4.59%
Long term rate of return on plan assets (in percents)	5.25%	5.25%
Long-term pay progression (in percents)	3.50%	3.60%
U.S. retiree health care plan
Weighted average assumptions used to determine benefit obligations [Abstract]
Discount rate (in percents)	4.07%	4.83%
Weighted average assumptions used to determine net periodic benefit cost [Abstract]
Discount rate (in percents)	4.83%	3.94%
Long term rate of return on plan assets (in percents)	4.50%	4.75%
Non-U.S. defined benefit plan
Weighted average assumptions used to determine benefit obligations [Abstract]
Discount rate (in percents)	2.34%	3.01%
Long-term pay progression (in percents)	3.27%	3.11%
Weighted average assumptions used to determine net periodic benefit cost [Abstract]
Discount rate (in percents)	3.01%	2.74%
Long term rate of return on plan assets (in percents)	3.75%	3.34%
Long-term pay progression (in percents)	3.11%	3.01%

The interest rate used to adjust for the time value of money for the plan.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (k)(1)

-URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920

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- Definition

Expected rate of compensation increases (for pay-related plans).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (k)(2)

-URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920

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- Definition

The interest rate used to adjust for the time value of money.

+ References

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- Definition

An assumption as to the rate of return on plan assets reflecting the average rate of earnings expected on the funds invested or to be invested to provide for the benefits included in the benefit obligation.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 30

-Glossary Expected Long-Term Rate of Return on Plan Assets

-URI http://asc.fasb.org/extlink&oid=6512013

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (k)(3)

-URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920

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- Definition

Expected rate of compensation increases (for pay-related plans).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (k)(2)

-URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920

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