Income taxes - Income tax expense (Details) - USD ($) $ in Millions		12 Months Ended
Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes
Applicable tax rate		26.00%	26.00%
Current tax expense:
Current period		$ 16.3	$ 20.3
Adjustment for prior periods		2.3	6.7
Total current tax		18.6	27.0
Deferred tax expense:
Origination and reversal of temporary differences		(38.0)	(4.4)
Change in unrecognized deductible temporary differences		5.2	1.4
Change in tax rate		0.3	(0.1)
Recognition of previously unrecognized tax losses		(122.4)
Total deferred tax expense		(154.9)	(3.1)
Income tax expense		$ (136.3)	$ 23.9
U.S.
Income Taxes
Applicable tax rate		35.00%
Forecast \| U.S.
Income Taxes
Applicable tax rate	21.00%

Adjustments of tax expense (income) recognised in the period for current tax of prior periods.

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The applicable income tax rate.

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The amount of income taxes payable (recoverable) in respect of the taxable profit (tax loss) for a period.

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The amount of current tax expense (income) and adjustments for the current tax of prior periods. [Refer: Current tax expense (income); Adjustments for current tax of prior periods]

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The amount of tax expense or income relating to changes in deferred tax liabilities and deferred tax assets, recognised in profit or loss. [Refer: Deferred tax assets; Deferred tax expense (income); Deferred tax liabilities]

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The amount of deferred tax expense or income relating to the creation or reversal of temporary differences. [Refer: Temporary differences [member]; Deferred tax expense (income)]

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The amount of deferred tax expense or income relating to tax rate changes or the imposition of new taxes. [Refer: Deferred tax expense (income)]

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The aggregate amount included in the determination of profit (loss) for the period in respect of current tax and deferred tax that relate to continuing operations. [Refer: Continuing operations [member]; Current tax expense (income); Deferred tax expense (income)]

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The amount of benefit arising from a previously unrecognised tax loss, tax credit or temporary difference of a prior period that is used to reduce deferred tax expense. [Refer: Deferred tax expense (income); Temporary differences [member]; Unused tax losses [member]; Unused tax credits [member]]

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The amount of benefit arising from a previously unrecognised temporary difference of a prior period that is used to reduce deferred tax expense. [Refer: Deferred tax expense (income)

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