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Accounting Policies - New Accounting Standards (Details) - USD ($) $ in Thousands | 6 Months Ended
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Jun. 30, 2018 | Jun. 30, 2017
Accounting Policies [Abstract] |  |
Excess Tax Benefits from Share-Based Compensation | $ 903 | $ 832
Payments Related to Tax Withholding for Share-based Compensation | $ 2,523 | $ 1,669

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- Definition Amount of cash outflow to satisfy an employee's income tax withholding obligation as part of a net-share settlement of a share-based award. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 25 -Paragraph 19A -URI http://asc.fasb.org/extlink&oid=96949009&loc=SL79513924-113897 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 15 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3291-108585 + Details
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