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Summary of Significant Accounting Policies Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions | 9 Months Ended |  |
--- | --- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2017 | Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss), Net of Tax | $ (18.9) | $ (20.3) | $ (23.2) | $ (29.3)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax | 1.2 | 1.8 |  |
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent |  | 9.0 |  |
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax |  | 6.7 |  |
Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member] | Pension Plan [Member] |  |  |  |
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax | (27.1) | (30.1) | (32.7) | (32.1)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax | 2.5 | 2.0 |  |
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent |  | 2.0 |  |
Other Comprehensive Income (Loss), before Reclassifications, before Tax |  | 0.0 |  |
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax |  | 0.0 |  |
Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member] | Other Postretirement Benefits Plan [Member] |  |  |  |
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax | 2.5 | 3.2 | 2.5 | 2.7
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax | 0.0 | 0.0 |  |
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent |  | 0.5 |  |
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost (Credit) [Member] | Pension Plan [Member] |  |  |  |
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax | 0.0 | 0.1 | 0.0 | 0.1
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax | 0.0 | 0.0 |  |
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent |  | 0.0 |  |
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost (Credit) [Member] | Other Postretirement Benefits Plan [Member] |  |  |  |
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax | 5.7 | 6.5 | $ 7.0 | $ 0.0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax | (1.3) | (0.2) |  |
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent |  | 6.5 |  |
Defined Benefit Plans Income Loss [Member] | Other Postretirement Benefits Plan [Member] |  |  |  |
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax |  | 0.0 |  |
Defined Benefit Plan Prior Service Cost [Member] | Other Postretirement Benefits Plan [Member] |  |  |  |
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax |  | 6.7 |  |
Settlement Cost [Member] |  |  |  |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax | 3.1 | 0.5 |  |
Settlement Cost [Member] | Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member] | Pension Plan [Member] |  |  |  |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax | 3.1 | 0.0 |  |
Settlement Cost [Member] | Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member] | Other Postretirement Benefits Plan [Member] |  |  |  |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax | 0.0 | 0.5 |  |
Settlement Cost [Member] | Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost (Credit) [Member] | Pension Plan [Member] |  |  |  |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax |  | 0.0 |  |
Settlement Cost [Member] | Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost (Credit) [Member] | Other Postretirement Benefits Plan [Member] |  |  |  |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax | $ 0.0 | $ 0.0 |  |

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