Equity incentive plans - Textuals (Details) - USD ($)	12 Months Ended
Dec. 30, 2017	Dec. 31, 2016	Dec. 26, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Tax benefit realized from stock options exercised	$ 11,100,000	$ 4,100,000	$ 13,100,000
Grant-date fair value, vested	9,900,000	10,600,000	7,400,000
Intrinsic value of options exercised in period	13,000,000	5,300,000	6,700,000
Unrecognized compensation costs related to stock options	12,700,000
Allocated Share-based compensation expense	$ 14,926,000	17,181,000	16,092,000
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, vesting period	4 years
Restricted shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated Share-based compensation expense	$ 0	40,000	198,000
Employee stock option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation costs, weighed average period	2 years 4 months 24 days
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant-date fair value, vested	$ 4,100,000	$ 3,500,000	$ 2,600,000
Unrecognized compensation costs related to stock options	$ 5,200,000
Unrecognized compensation costs, weighed average period	1 year 9 months
Granted, weighted average grant-date fair value (in dollars per share)	$ 52.44	$ 44.34	$ 46.21
Granted	91,272
Allocated Share-based compensation expense	$ 4,337,000	$ 3,608,000	$ 3,408,000
Performance shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation costs, weighed average period	1 year 11 months 8 days
Granted, weighted average grant-date fair value (in dollars per share)	$ 59.03
Granted	84,705	121,030
Unrecognized compensation cost, nonvested awards	$ 4,000,000
Allocated Share-based compensation expense	$ 1,277,000	$ 1,086,000	$ 0
Award vesting rights, percentage	200.00%
Equity Incentive Plan 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, weighted average grant-date fair value (in dollars per share)		$ 47.39	$ 51.67
Granted		21,101	27,096
Unrecognized compensation cost, nonvested awards	$ 0
Allocated Share-based compensation expense	$ 150,000
Grant-date fair value, vested		$ 1,400,000
Equity Incentive Plan 2011 \| Employee stock option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares, Granted	1,181,777
Allocated Share-based compensation expense	$ 8,611,000	$ 10,654,000	$ 10,519,000
Equity Incentive Plan 2011 \| Employee stock option \| Common stock \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award requisite service period	4 years
Maximum contractual term	7 years
Equity Incentive Plan 2011 \| Employee stock option \| Common stock \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum contractual term	10 years
Equity Incentive Plan 2011 \| Performance shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation costs, weighed average period	1 year
Granted, weighted average grant-date fair value (in dollars per share)			$ 37.94
Granted			150,000
Unrecognized compensation cost, nonvested awards	$ 1,200,000
Performance based shares based on shareholder return \| Performance shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, weighted average grant-date fair value (in dollars per share)	$ 67.52	$ 55.36
Granted	37,027	39,684
Performance based shares subject to service condition and performance vesting condition \| Performance shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, weighted average grant-date fair value (in dollars per share)	$ 52.44	$ 44.22
Granted	47,678	81,346
Nonexecutive \| Stock Incentive Plan 2006 \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum shares of common stock that may be delivered in satisfaction of awards (in shares)			6,200,000

Share-based Compensation Arrangement by Share-based Payment Award, Maximum Contractual Term

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Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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Unrecognized cost of unvested share-based compensation awards.

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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Unrecognized cost of unvested options awarded to employees as compensation.

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Disclosure of the aggregate tax benefit realized from the exercise of stock options and the conversion of similar instruments during the annual period.

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Estimated period over which an employee is required to provide service in exchange for the equity-based payment award, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

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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.

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The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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Gross number of share options (or share units) granted during the period.

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No definition available.

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Percentage of vesting of share-based compensation awards.

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Fair value of options vested. Excludes equity instruments other than options, for example, but not limited to, share units, stock appreciation rights, restricted stock.

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