Income taxes - Reconciliation of the libaility for uncertain tax positions (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 26, 2014	Dec. 27, 2013
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits at the beginning of the year	$ 3,522us-gaap_UnrecognizedTaxBenefits	$ 7,880us-gaap_UnrecognizedTaxBenefits
Additions to tax positions related to current year	628	1,202
Additions to tax positions related to prior years	28	0
Reductions to tax positions related to prior years	(24)	(1,058)
Settlements with tax authorities	(91)	0
Lapses in statutes of limitation	(1,677)	(4,502)
Unrecognized tax benefits at the end of the year	$ 2,386us-gaap_UnrecognizedTaxBenefits	$ 3,522us-gaap_UnrecognizedTaxBenefits

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Amount of unrecognized tax benefits pertaining to uncertain tax positions taken in tax returns.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Glossary Unrecognized Tax Benefit

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Amount of decrease in unrecognized tax benefits resulting from tax positions taken in prior period tax returns.

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Amount of decrease in unrecognized tax benefits resulting from settlements with taxing authorities.

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Amount of increase in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period tax return.

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Amount of increase in unrecognized tax benefits resulting from tax positions taken in prior period tax returns.

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Amount of decrease in unrecognized tax benefits resulting from lapses of applicable statutes of limitations.

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