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Share-Based Compensation Plans - Schedule of Performance-based plans (Details) - Performance Shares - USD ($) $ / shares in Units, shares in Thousands, $ in Millions | 6 Months Ended | 12 Months Ended
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Jun. 30, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Target PSAs granted during period (in shares) | 564 | 548 | 750
Weighted average fair value per share at date of grant (in dollars per share) | $ 134 | $ 114 | $ 100
Number of shares that would be issued based on current performance levels (in shares) | 564 | 941 | 742
Unamortized expense, based on current performance levels | $ 68 | $ 58 | $ 12

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- Definition Represents the number of shares to be issued for performance-based stock award programs, based on the current performance levels. + References No definition available. + Details
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- Definition Represents unamortized expense of performance-based stock award plans, based on current performance levels. + References No definition available. + 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://fasb.org/us-gaap/role/ref/legacyRef -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=109197908&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://fasb.org/us-gaap/role/ref/legacyRef -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=109197908&loc=d3e5070-113901 + Details
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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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