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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Discount rate from fair value on offering date that participants pay for shares.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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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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The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The risk-free interest rate assumption that is used in valuing an option on its own shares.

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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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No definition available.

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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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Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
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-Publisher FASB
-Name Accounting Standards Codification
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Purchase price of common stock expressed as a percentage of its fair value.

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Number shares or portion of an award that are no longer contingent on satisfaction of either a service condition, market condition or a performance condition, thereby giving the employee the legal right to convert the award to shares, to sell the shares, and be entitled to the cash proceeds of such sale, for awards vesting over the maximum vesting period. Vesting may be expressed as being 25 percent of the shares under option on each anniversary of the grant date.

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Percentage of annual incentive bonus that may be used to purchase RSU's.

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Represents the number of stock incentive plans maintained by the entity.

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Stock-Based Compensation (Details)	9 Months Ended		12 Months Ended
Stock-Based Compensation (Details)	Sep. 30, 2018 item $ / shares shares	Oct. 01, 2017 $ / shares shares	Dec. 31, 2017
Stock-based compensation
Number of stock incentive plans \| item	1
Second Amended and Restated 2004 Stock Incentive Plan \| Restricted stock
Stock-based compensation
Granted (in shares)	140,937	127,960
Second Amended and Restated 2004 Stock Incentive Plan \| Restricted stock \| Maximum
Stock-based compensation
Vesting period	3 years
Vesting rate per year for maximum vesting period	0.33
Second Amended and Restated 2004 Stock Incentive Plan \| Deferred shares
Stock-based compensation
Vesting period	3 years
Vesting rate per year for maximum vesting period	0.33
Second Amended and Restated 2004 Stock Incentive Plan \| Performance stock units
Stock-based compensation
Granted (in shares)	96,128	98,812
Management Stock Purchase Plan \| Maximum
Stock-based compensation
Percentage of annual incentive bonus that may be used to purchase RSU's	50.00%
Management Stock Purchase Plan \| Class A
Stock-based compensation
Purchase price as percentage of fair market value of common stock on grant date	80.00%
Shares authorized	2,000,000
Management Stock Purchase Plan \| Restricted stock units (RSUs)
Stock-based compensation
Discount percentage from fair value for acquiring shares	20.00%
Granted (in shares)	36,208	47,222
Fair value assumptions
Expected life (years)	3 years		3 years
Expected stock price volatility (as a percent)	24.10%		25.00%
Expected dividend yield (as a percent)	1.00%		1.20%
Risk-free interest rate (as a percent)	2.40%		1.50%
Weighted average grant-date fair value (in dollars per share) \| $ / shares	$ 21.80	$ 16.84
Management Stock Purchase Plan \| Restricted stock units (RSUs) \| Minimum
Stock-based compensation
Vesting period	3 years