Postretirement benefit plans - Health care cost trend rates (Details) (U.S. retiree health care plan)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
U.S. retiree health care plan
Defined Benefit Plan Disclosure [Line Items]
Assumed health care cost trend rate for next year (in percents)	7.00%	7.00%
Ultimate trend rate (in percents)	5.00%	5.00%
Year in which ultimate trend rate is reached	2023	2022

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The assumed health care cost trend rate for the next year used to measure the expected cost of benefits covered by the plan (gross eligible charges). This is based upon the annual rate of change in the cost of health care benefits currently provided by the postretirement benefit plan, due to factors other than changes in the composition of the plan population by age and dependency status.

+ 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 (l)

-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

-Glossary Health Care Cost Trend Rate

-URI http://asc.fasb.org/extlink&oid=6514820

+ Details

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- Definition

The ultimate trend rate for health care costs.

+ References

+ Details

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- Definition

Year the ultimate health care cost trend rate is expected to be reached, in CCYY format.

+ References

+ Details

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