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Employee plans - Schedule of Weighted-Average Assumptions (Details)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Pension Plans | U.S. Plans      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate 4.38% 4.56% 4.09%
Expected long-term rate of return on assets 6.80% 6.80% 6.80%
Weighted-average assumptions used to determine benefit obligations at December 31 were as follows:      
Discount rate 3.87% 4.38% 4.56%
Pension Plans | International Plans      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate 2.34% 3.30% 3.05%
Rate of compensation increase 1.66% 1.66% 1.68%
Expected long-term rate of return on assets 5.30% 5.99% 6.45%
Weighted-average assumptions used to determine benefit obligations at December 31 were as follows:      
Discount rate 2.24% 2.34% 3.30%
Rate of compensation increase 1.55% 1.66% 1.68%
Other Postretirement Benefit Plans      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate 5.53% 5.61% 5.27%
Expected long-term rate of return on assets 5.50% 5.50% 5.50%
Weighted-average assumptions used to determine benefit obligations at December 31 were as follows:      
Discount rate 5.30% 5.53% 5.61%