Summary of significant accounts policies - Property, Plant, Equipment, and Software (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 26, 2014	Dec. 27, 2013
Property, Plant and Equipment [Line Items]
Capitalized software costs during period	$ 1.2	$ 2.2
Capitalized computer software, net	$ 8.7us-gaap_CapitalizedComputerSoftwareNet	$ 8.8us-gaap_CapitalizedComputerSoftwareNet
Buildings and Improvements [Member] \| Minimum [Member]
Property, Plant and Equipment [Line Items]
Useful lives	5 years
Buildings and Improvements [Member] \| Maximum [Member]
Property, Plant and Equipment [Line Items]
Useful lives	30 years
Machinery and Equipment [Member] \| Minimum [Member]
Property, Plant and Equipment [Line Items]
Useful lives	2 years
Machinery and Equipment [Member] \| Maximum [Member]
Property, Plant and Equipment [Line Items]
Useful lives	10 years
Software [Member] \| Minimum [Member]
Property, Plant and Equipment [Line Items]
Useful lives	3 years
Software [Member] \| Maximum [Member]
Property, Plant and Equipment [Line Items]
Useful lives	7 years

Additions made to capitalized computer software costs during the period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 30

-Section 50

-Paragraph 1

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16265-109275

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The carrying amount of capitalized computer software costs net of accumulated amortization as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 985

-SubTopic 20

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6501960&loc=d3e128487-111756

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 985

-SubTopic 20

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6501960&loc=d3e128462-111756

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

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Namespace Prefix:
Data Type:	na
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- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_MachineryAndEquipmentMember
Namespace Prefix:
Data Type:	na
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Period Type: