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Equity-Based Compensation (Performance Stock Units - Settled in Cash) (Details) - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2018 | Dec. 31, 2016 | Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Accrued equity based compensation | $ 38.5 |  |
Performance stock units |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Typical vesting period, in years | 3 years |  |
Performance period | 1 year |  |
Percentage of target performance stock units earned |  | 200.00% | 200.00%
Aggregate shares earned (in shares) |  | 256,000 | 438,000
Targeted award (in shares) |  | 128,000 | 219,000
Non Current Liabilities |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Accrued equity based compensation | $ 23.0 |  |

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- Definition Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + 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 (a)(1) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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