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Equity-Based Compensation (Details) - USD ($) $ / shares in Units, $ in Millions |  |  | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Aug. 01, 2018 | Jun. 26, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2017 | Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Number of options vested (in shares) | 456,613 |  |  |  |  |
Weighted-average grant-date fair value (in usd per share) | $ 35.93 |  |  |  |  |
Payment of incentive compensation plan withholding taxes |  |  | $ 27.1 | $ 42.0 |  |
Employee Stock Option |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Payment of incentive compensation plan withholding taxes |  |  | $ 19.0 |  |  |
Performance Shares |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Number of awards vested in the period (in shares) |  |  |  |  | 321,880 | 748,855
Potential vesting percentage range of shares |  |  |  |  | 192.00% | 193.50%
Awards vested, weighted average grant date fair value (in usd per share) |  |  |  |  | $ 37.84 | $ 24.40
Payment of incentive compensation plan withholding taxes |  |  |  |  | $ 9.0 | $ 18.0
Restricted Stock Units (RSUs) |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Number of awards vested in the period (in shares) |  | 997,450 |  |  |  |
Awards vested, weighted average grant date fair value (in usd per share) |  | $ 17.04 |  |  |  |
Payment of incentive compensation plan withholding taxes |  |  |  | $ 24.0 |  |

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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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 15 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3291-108585 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 25 -Paragraph 19A -URI http://asc.fasb.org/extlink&oid=107668666&loc=SL79513924-113897 + Details
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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period. + 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)(2) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement. + 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)(2) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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