Income taxes - Reconciliation of Changes in Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at January 1	$ 18.9	$ 27.9	$ 35.8
Additions based on tax positions related to the current year	11.1	2.6	4.2
Changes for tax positions of prior years	(0.6)	(6.1)	(11.2)
Reductions as a result of a lapse of applicable statute of limitations	(2.3)	(2.4)	(2.7)
Settlements	(1.1)	(1.5)	(0.2)
Changes due to foreign currency	(0.4)	(1.6)	2.0
Balance at December 31	$ 25.6	$ 18.9	$ 27.9