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Share-Based Payments - Share-based Payments - Non-Employee Directors RSUs (Details) - Restricted Share Units - shares | 3 Months Ended
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Mar. 31, 2018 | Mar. 31, 2017
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items] |  |
RSUs granted, number of shares (in shares) | 221,943 | 174,711
Non-Employee Directors |  |
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items] |  |
RSUs granted, number of shares (in shares) | 29,025 | 24,456
Board of Directors Chairman |  |
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items] |  |
RSUs granted, number of shares (in shares) | 12,900 | 10,962

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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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