Employee plans - Schedule of Weighted-Average Assumptions (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Plans | U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.87%	4.38%	4.56%
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	4.50%	3.87%	4.38%
Pension Plans | International Plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate	2.24%	2.34%	3.30%
Expected long-term rate of return on assets	4.38%	5.30%	5.99%
Weighted-average assumptions used to determine benefit obligations at December 31 were as follows:
Discount rate	2.55%	2.24%	2.34%
Other Postretirement Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate	5.30%	5.53%	5.61%
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	5.67%	5.30%	5.53%