Income taxes - Reconciliation items from U.S. statutory income tax rate to the effective tax rate (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
U.S. statutory income tax rate	35.00%	35.00%	35.00%
U.S. Tax Act	12.70%
U.S. excess tax benefit for stock compensation	(4.10%)	(3.00%)
Non-U.S. effective tax rates	(2.50%)	(3.70%)	(4.00%)
U.S. tax benefit for manufacturing	(1.60%)	(1.50%)	(1.60%)
U.S. R&D tax credit	(1.10%)	(1.20%)	(1.30%)
Impact of changes to uncertain tax positions	0.70%	0.60%	0.20%
U.S. non-deductible expenses	0.20%	0.30%	0.30%
Other	0.10%	0.60%	0.60%
Effective tax rate	39.40%	27.10%	29.20%