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Summary of Significant Accounting Policies Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2019 | Mar. 31, 2018 | Dec. 31, 2018 | Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss), Net of Tax | $ (22.1) | $ (23.0) | $ (28.9) | $ (23.2)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax | 0.2 | 0.2 |  |
Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member] | Pension Plan [Member] |  |  |  |
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax | (31.5) | (32.0) | (38.8) | (32.7)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax | 0.7 | 0.7 |  |
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 | 4.6 | 2.5 | 4.6 | 2.5
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax | 0.0 | 0.0 |  |
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 | 0.0 | 0.0 | 0.0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax | 0.0 | 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 | 4.8 | 6.5 | $ 5.3 | $ 7.0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax | (0.5) | $ (0.5) |  |
Settlement Cost [Member] |  |  |  |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax | 6.6 |  |  |
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 | 6.6 |  |  |
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 |  |  |
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 |  |  |

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