Income taxes - Income Tax Rate Reconciliation (Details)	12 Months Ended
Dec. 30, 2017	Dec. 31, 2016	Dec. 26, 2015
Income Tax Disclosure [Abstract]
Computed federal income tax expense, at statutory rate	35.00%	35.00%	35.00%
Impairment of investment in Japan JV	0.00%	0.00%	9.40%
State income taxes	5.50%	5.10%	6.60%
Benefits and taxes related to foreign operations	(1.40%)	(2.70%)	(4.40%)
Excess tax benefits	(2.30%)	0.00%	0.00%
Change in valuation allowance	2.60%	0.00%	0.00%
Other permanent differences	(0.40%)	0.10%	0.60%
Impact of Tax Act	(42.30%)	0.00%	0.00%
Other, net	(0.10%)	0.10%	0.60%
Effective tax rate	(3.40%)	37.60%	47.80%

Effective Income Tax Rate Reconciliation, Share-based Compensation, Excess Tax Benefit, Percent

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

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Percentage 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 changes in the valuation allowance for deferred tax assets.

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Percentage 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 applicable to statutory income tax expense (benefit) outside of the country of domicile.

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Percentage 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 applicable to state and local income tax expense (benefit), net of federal tax expense (benefit).

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