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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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Number of shares (or other type of equity) issued during the period as a result of any equity-based compensation plan other than an employee stock ownership plan (ESOP), net of any shares forfeited. Shares issued could result from the issuance of restricted stock, the exercise of stock options, stock issued under employee stock purchase plans, and/or other employee benefit plans.

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Represents the aggregate amount of shares that have been forfeited and returned to the pool of shares which could be granted under the plan.

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Stock-Based Compensation Plans - 2007 Stock Incentive Plan (Details) - shares		12 Months Ended
	May 28, 2015	Dec. 31, 2017	May 20, 2010	May 24, 2007
The 2007 plan
Stock-based compensation plans
Number of shares authorized	2,800,000			2,000,000
Additional shares authorized	800,000
Restricted stock, forfeited, and returned to pool		166,744
Number of shares available		350,784
Employee Stock Purchase Plan
Stock-based compensation plans
Number of shares authorized			250,000
Restricted stock \| The 2007 plan
Stock-based compensation plans
Number of shares awarded		2,615,960
Minimum \| Restricted stock \| The 2007 plan
Stock-based compensation plans
Vesting period	1 year
Maximum \| Restricted stock \| The 2007 plan
Stock-based compensation plans
Vesting period	4 years