Business overview and basis of presentation (Components of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
	Jan. 01, 2018	Sep. 27, 2019	Sep. 28, 2018	Sep. 27, 2019	Sep. 28, 2018
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	$ 0.6	$ (85.4)	$ (53.9)	$ (78.2)	$ 0.6
Increase (decrease)		(54.2)	(17.3)	(60.8)	(72.0)
Income tax impact		(1.3)	0.0	(1.3)	0.0
Other comprehensive income (loss) before reclassifications, net of income taxes		(55.5)	(17.3)	(62.1)	(72.0)
Increase (decrease)			(1.0)	(0.9)	(0.5)
Income tax impact			0.2	0.3	0.1
Amounts reclassified from accumulated other comprehensive income (loss), net of income taxes			(0.8)	(0.6)	(0.4)
Net current period other comprehensive income (loss)		(54.9)	(17.3)	(61.5)	(72.0)
Net current period other comprehensive income (loss)		0.0	0.8	0.6	0.4
Total other comprehensive loss, net of income taxes		(55.5)	(18.1)	(62.7)	(72.4)
Adoption of accounting standards	(0.2)
Ending balance	0.4	(140.9)	(72.0)	(140.9)	(72.0)
Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	10.9	(80.9)	(43.8)	(74.3)	10.9
Increase (decrease)		(53.5)	(17.3)	(60.1)	(72.0)
Income tax impact		(1.4)	0.0	(1.4)	0.0
Other comprehensive income (loss) before reclassifications, net of income taxes		(54.9)	(17.3)	(61.5)	(72.0)
Increase (decrease)			0.0	0.0	0.0
Income tax impact			0.0	0.0	0.0
Amounts reclassified from accumulated other comprehensive income (loss), net of income taxes			0.0	0.0	0.0
Net current period other comprehensive income (loss)		(54.9)	(17.3)		(72.0)
Adoption of accounting standards	0.0
Ending balance	10.9	(135.8)	(61.1)	(135.8)	(61.1)
Cash Flow Hedge Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	0.0	0.0	0.0	0.0	0.0
Increase (decrease)		(0.7)	0.0	(0.7)	0.0
Income tax impact		0.1	0.0	0.1	0.0
Other comprehensive income (loss) before reclassifications, net of income taxes		(0.6)	0.0	(0.6)	0.0
Increase (decrease)			0.0	0.0	0.0
Income tax impact			0.0	0.0	0.0
Amounts reclassified from accumulated other comprehensive income (loss), net of income taxes			0.0	0.0	0.0
Net current period other comprehensive income (loss)		(0.6)	0.0	(0.6)	0.0
Adoption of accounting standards	0.0
Ending balance	0.0	(0.6)	0.0	(0.6)	0.0
Pension Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(10.3)	(4.5)	(10.1)	(3.9)	(10.3)
Increase (decrease)		0.0	0.0	0.0	0.0
Income tax impact		0.0	0.0	0.0	0.0
Other comprehensive income (loss) before reclassifications, net of income taxes		0.0	0.0	0.0	0.0
Increase (decrease)			(1.0)	(0.9)	(0.5)
Income tax impact			0.2	0.3	0.1
Amounts reclassified from accumulated other comprehensive income (loss), net of income taxes			(0.8)	(0.6)	(0.4)
Net current period other comprehensive income (loss)		0.0	(0.8)		0.4
Adoption of accounting standards	(0.2)
Ending balance	$ (10.5)	$ (4.5)	$ (10.9)	$ (4.5)	$ (10.9)