Income taxes (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 29, 2018	Dec. 30, 2017	Sep. 30, 2017	Sep. 29, 2018	Sep. 30, 2017
Income Tax Disclosure [Abstract]
Income tax expense	$ 7.2		$ 15.9	$ 30.4	$ 32.9
Pre-tax income	$ 74.2		$ 34.0	$ 219.7	$ 85.3
Effective income tax rate	9.70%		46.80%	13.80%	38.60%
Decrease in effective tax rate - discrete items	$ 5.7
Provisional income tax benefit		$ 118.2
Provisional income tax benefit - revaluation of deferred tax liabilities		153.7
Measurement period adjustment, income tax expense - revaluation of deferred tax liabilities	0.8
Provisional income tax expense - repatriation of foreign earnings		$ 33.6
Measurement period adjustment, income tax benefit - repatriation of foreign earnings	$ 3.0