Restructuring initiatives - Restructuring Reserve Activity (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jul. 01, 2017
Restructuring Reserve [Roll Forward]
Balance as of the beginning of the period	$ 8.6	$ 5.0
Net charge for the period	2.3	4.6
Utilized during the period	(6.3)	(5.3)
Released during the period	(0.3)	(0.1)
Foreign currency translation	(0.1)	0.1
Balance as of the end of the period	$ 4.2	$ 4.3