Income taxes - Reconciliation items from U.S. statutory income tax rate to the effective tax rate (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
U.S. statutory income tax rate	21.00%	21.00%	21.00%
U.S. tax benefit for foreign derived intangible income	(6.10%)	(4.90%)	(5.30%)
Impact of changes in uncertain tax positions	(4.00%)	(0.10%)	0.00%
U.S. excess tax benefit for stock compensation	(2.50%)	(3.10%)	(2.00%)
U.S. R&D tax credit	(1.30%)	(1.40%)	(1.30%)
U.S. Tax Act transitional non-cash expense	0.00%	0.00%	4.20%
Other	(0.10%)	0.90%	(0.10%)
Effective tax rate	7.00%	12.40%	16.50%