Postretirement benefit plans - Weighted average allocations (Details)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
U.S. defined benefit plan \| Fixed income securities and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Targeted allocation (in percents)	65.00%
Actual plan asset allocation (in percents)	65.00%	65.00%
U.S. defined benefit plan \| Equity securities
Defined Benefit Plan Disclosure [Line Items]
Targeted allocation (in percents)	35.00%
Actual plan asset allocation (in percents)	35.00%	35.00%
U.S. retiree health care plan \| Fixed income securities and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Targeted allocation (in percents)	50.00%
Actual plan asset allocation (in percents)	49.00%	49.00%
U.S. retiree health care plan \| Equity securities
Defined Benefit Plan Disclosure [Line Items]
Targeted allocation (in percents)	50.00%
Actual plan asset allocation (in percents)	51.00%	51.00%
Non-U.S. defined benefit plan \| Fixed income securities and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Targeted allocation range minimum (in percents)	60.00%
Targeted allocation range maximum (in percents)	100.00%
Actual plan asset allocation (in percents)	73.00%	70.00%
Non-U.S. defined benefit plan \| Equity securities
Defined Benefit Plan Disclosure [Line Items]
Targeted allocation range minimum (in percents)	0.00%
Targeted allocation range maximum (in percents)	40.00%
Actual plan asset allocation (in percents)	27.00%	30.00%

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Target allocation of investments of this type to total plan assets presented on a weighted average basis.

+ 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 (d)(i)

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

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 5

-Subparagraph (c)(i)

-URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e2446-114920

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- Definition

Target allocation maximum percentage of investments of this type to total plan assets presented on a weighted average basis.

+ References

+ Details

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- Definition

Target allocation minimum percentage of investments of this type to total plan assets presented on a weighted average basis.

+ References

+ Details

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- Definition

Percentage of fair value of investments (categorized by debt securities, equity securities, real estate and other plan assets) to the fair value of plan assets held.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 5

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e2446-114920

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (d)(1)

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

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