Income taxes - Narratives (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 28, 2020	Mar. 30, 2019
Operating Loss Carryforwards
Income tax benefit	$ (16.1)	$ (539.7)
Income from continuing operations before income taxes	$ 23.8	$ 65.4
Reported effective income tax rate (percent)	(67.60%)	(825.20%)
Change in valuation allowance		$ (617.3)
Discrete tax expense (benefit)	$ (24.7)	$ 66.1
Effect of change in tax laws	6.3
India
Operating Loss Carryforwards
Discrete tax expense (benefit)	$ (3.2)

Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Amount of income (loss) from continuing operations, including income (loss) from equity method investments, before deduction of income tax expense (benefit), and income (loss) attributable to noncontrolling interest.

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to other adjustments.

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Amount of other income tax expense (benefit).

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Amount of increase (decrease) in the valuation allowance for a specified deferred tax asset.

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