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Stock-Based Compensation (Details) | 9 Months Ended | 12 Months Ended
--- | --- | ---
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 |  |

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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition 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 + Details
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- Definition 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). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition 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). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(iii) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(ii) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The risk-free interest rate assumption that is used in valuing an option on its own 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 -Subparagraph (f)(2)(iv) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition 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). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition 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 -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 14.D.2) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 + Details
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- Definition 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. + References No definition available. + Details
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