Employee plans - Company's Investment Policy Targets and Ranges for Each Asset Category (Details)	12 Months Ended
Dec. 31, 2016
U.S. Plans \| Equity securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan target allocation percentage of assets	58.00%
Defined benefit plan target plan asset allocations range, minimum	52.00%
Defined benefit plan target plan asset allocations range, maximum	64.00%
U.S. Plans \| Debt securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan target allocation percentage of assets	36.00%
Defined benefit plan target plan asset allocations range, minimum	33.00%
Defined benefit plan target plan asset allocations range, maximum	39.00%
U.S. Plans \| Real estate
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan target allocation percentage of assets	6.00%
Defined benefit plan target plan asset allocations range, minimum	3.00%
Defined benefit plan target plan asset allocations range, maximum	9.00%
U.S. Plans \| Cash
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan target allocation percentage of assets	0.00%
Defined benefit plan target plan asset allocations range, minimum	0.00%
Defined benefit plan target plan asset allocations range, maximum	5.00%
U.S. Plans \| Other
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan target allocation percentage of assets	0.00%
Defined benefit plan target plan asset allocations range, minimum	0.00%
International Plans \| Equity securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan target allocation percentage of assets	29.00%
Defined benefit plan target plan asset allocations range, minimum	23.00%
Defined benefit plan target plan asset allocations range, maximum	35.00%
International Plans \| Debt securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan target allocation percentage of assets	55.00%
Defined benefit plan target plan asset allocations range, minimum	48.00%
Defined benefit plan target plan asset allocations range, maximum	61.00%
International Plans \| Real estate
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan target allocation percentage of assets	1.00%
Defined benefit plan target plan asset allocations range, minimum	0.00%
Defined benefit plan target plan asset allocations range, maximum	3.00%
International Plans \| Cash
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan target allocation percentage of assets	1.00%
Defined benefit plan target plan asset allocations range, minimum	0.00%
Defined benefit plan target plan asset allocations range, maximum	5.00%
International Plans \| Other
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan target allocation percentage of assets	14.00%
Defined benefit plan target plan asset allocations range, minimum	7.00%
Defined benefit plan target plan asset allocations range, maximum	21.00%

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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 5
-Subparagraph (c)(i)
-URI http://asc.fasb.org/extlink&oid=65897934&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)(i)
-URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920

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

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Target allocation minimum percentage 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=65897934&loc=d3e1928-114920

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-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e2446-114920

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