Income taxes - Reconciliation of Changes in Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at January 1	$ 27.7	$ 35.0	$ 26.3
Additions based on tax positions related to the current year	2.7	3.4	14.4
Changes for tax positions of prior years	2.0	(4.0)	(1.4)
Reductions as a result of a lapse of applicable statute of limitations	(2.8)	(3.4)	(1.6)
Settlements	(0.1)	(0.9)	(0.9)
Changes due to foreign currency	(3.7)	(2.4)	(1.8)
Balance at December 31	$ 25.8	$ 27.7	$ 35.0