Retirement plans - Textual (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 30, 2017	Dec. 31, 2016	Dec. 26, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum annual contribution percentage, 401(k)	80.00%
Employer contributions 401(k)	$ 3.4	$ 3.3	$ 3.2
NQDC maximum pretax contribution percentage	50.00%
NQDC plan liability	$ 13.5	11.1
NQDC investments held	$ 10.8	$ 9.3
Defined Benefit Plan, Effect of Settlements and Curtailments on Accumulated Benefit Obligation			$ 4.1
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employer match percentage 401(k)	4.00%	4.00%	4.00%

Deferred Compensation Arrangement with Individual, Maximum Employee Contribution Percentage

No definition available.

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- Definition

The carrying amount of the liability as of the balance sheet date to an individual under a deferred compensation arrangement. This amount may be the result of periodic accruals made over the period of active employment, or reflect termination benefits resulting contractual terms or a death benefit.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 710
-SubTopic 10
-Section 30
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6409875&loc=d3e20015-108363

+ Details

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- Definition

Carrying amount as of the balance sheet date of assets held under deferred compensation agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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- Definition

For pension plans, the amount of Increase or Decrease in the accumulated benefit obligation related to the effects of a settlement or curtailment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (r)
-URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920

+ Details

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- Definition

The amount of the cost recognized during the period for defined contribution plans.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 70
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942

+ Details

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- Definition

Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.

+ References

No definition available.

+ Details

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- Definition

Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan.

+ References

No definition available.

+ Details

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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