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Summary of Significant Accounting Policies Discussion Adoption of ASU 2017-07 (Details) - USD ($) $ in Millions |  | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Apr. 01, 2017 | Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Apr. 01, 2017 | Mar. 31, 2017 | Dec. 31, 2017 | Dec. 31, 2018 | Dec. 31, 2016
Successor |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Operating costs and expenses |  |  |  |  |  |  |  |  |  |  | $ 3,052.7 | $ 4,072.6 |
Selling and administrative expenses |  |  |  |  |  |  |  |  |  |  | 106.3 | 158.1 |
Net mark-to-market adjustment on actuarially determined liabilities, in Operating Profit |  |  |  |  |  |  |  |  |  |  | 0.0 |  |
Operating profit (loss) |  | $ 126.8 | $ 130.3 | $ 165.3 | $ 239.2 | $ 301.7 | $ 209.5 | $ 152.6 |  |  | 663.8 | 661.6 |
Net periodic benefit costs, excluding service cost |  |  |  |  |  |  |  |  |  |  | 21.9 | 18.1 |
Net mark-to-market adjustment on actuarially determined liabilities |  | $ 125.5 |  |  |  | $ 45.2 |  |  |  |  | (45.2) | (125.5) |
Income (loss) from continuing operations before income taxes |  |  |  |  |  |  |  |  |  |  | 552.1 | 664.1 |
Adjustment |  |  |  |  |  |  |  |  |  |  |  | $ 22.5 |
Successor | Operating costs and expenses |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Adjustment |  |  |  |  |  |  |  |  |  |  | (22.8) |  |
Successor | Selling and administrative expenses |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Adjustment |  |  |  |  |  |  |  |  |  |  | 0.9 |  |
Successor | Net mark-to-market adjustment on actuarially determined liabilities, Operating Profit |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Adjustment |  |  |  |  |  |  |  |  |  |  | 45.2 |  |
Successor | Operating profit (loss) |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Adjustment |  |  |  |  |  |  |  |  |  |  | (23.3) |  |
Successor | Net periodic benefit costs, excluding service cost |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Adjustment |  |  |  |  |  |  |  |  |  |  | 21.9 |  |
Successor | Net mark-to-market adjustment on actuarially determined liabilities |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Adjustment |  |  |  |  |  |  |  |  |  |  | (45.2) |  |
Successor | Income (loss) from continuing operations before income taxes |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Adjustment |  |  |  |  |  |  |  |  |  |  | 0.0 |  |
Successor | Before Application of Accounting Guidance |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Operating costs and expenses |  |  |  |  |  |  |  |  |  |  | 3,075.5 |  |
Selling and administrative expenses |  |  |  |  |  |  |  |  |  |  | 105.4 |  |
Net mark-to-market adjustment on actuarially determined liabilities, in Operating Profit |  |  |  |  |  |  |  |  |  |  | (45.2) |  |
Operating profit (loss) |  |  |  |  |  |  |  |  |  |  | 687.1 |  |
Net periodic benefit costs, excluding service cost |  |  |  |  |  |  |  |  |  |  | 0.0 |  |
Net mark-to-market adjustment on actuarially determined liabilities |  |  |  |  |  |  |  |  |  |  | 0.0 |  |
Income (loss) from continuing operations before income taxes |  |  |  |  |  |  |  |  |  |  | $ 552.1 |  |
Predecessor |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Operating costs and expenses |  |  |  |  |  |  |  |  | $ 950.2 |  |  |  | $ 4,070.0
Selling and administrative expenses |  |  |  |  |  |  |  |  | 36.3 |  |  |  | 149.4
Operating profit (loss) | $ 0.0 |  |  |  |  |  |  |  | 212.5 | $ 212.5 |  |  | (219.8)
Net periodic benefit costs, excluding service cost |  |  |  |  |  |  |  |  | 14.4 |  |  |  | 57.1
Net mark-to-market adjustment on actuarially determined liabilities |  |  |  |  |  |  |  |  | 0.0 |  |  |  | 0.0
Income (loss) from continuing operations before income taxes |  |  |  |  |  |  |  |  | (459.3) |  |  |  | (758.3)
Predecessor | Operating costs and expenses |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Adjustment |  |  |  |  |  |  |  |  | (13.5) |  |  |  | (53.1)
Predecessor | Selling and administrative expenses |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Adjustment |  |  |  |  |  |  |  |  | (0.9) |  |  |  | (4.0)
Predecessor | Operating profit (loss) |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Adjustment |  |  |  |  |  |  |  |  | 14.4 |  |  |  | 57.1
Predecessor | Net periodic benefit costs, excluding service cost |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Adjustment |  |  |  |  |  |  |  |  | 14.4 |  |  |  | 57.1
Predecessor | Income (loss) from continuing operations before income taxes |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Adjustment |  |  |  |  |  |  |  |  | 0.0 |  |  |  | 0.0
Predecessor | Before Application of Accounting Guidance |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Operating costs and expenses |  |  |  |  |  |  |  |  | 963.7 |  |  |  | 4,123.1
Selling and administrative expenses |  |  |  |  |  |  |  |  | 37.2 |  |  |  | 153.4
Operating profit (loss) |  |  |  |  |  |  |  |  | 198.1 |  |  |  | (276.9)
Net periodic benefit costs, excluding service cost |  |  |  |  |  |  |  |  | 0.0 |  |  |  | 0.0
Income (loss) from continuing operations before income taxes |  |  |  |  |  |  |  |  | $ (459.3) |  |  |  | $ (758.3)

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