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Stock-based Compensation (Fully-vested Stock Awards Non-employees) (Details) - Performance Stock Units - shares | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
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Share-based Compensation Arrangement by Share-based Payment Award |  |  |
Number of shares available for future grant - Beginning of year | 675,447 | 407,307 | 469,840
Number of shares available for future grant - End of year | 639,390 | 675,447 | 407,307

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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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- Definition 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. + 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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