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Components of accumulated other comprehensive loss (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |  |
Other comprehensive loss before income tax effect | $ (95.2) |  | $ (95.2) |  | $ (99.4)
Income tax effect | 6.5 |  | 6.5 |  | 7.7
Accumulated other comprehensive loss | (88.7) |  | (88.7) |  | (91.7)
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |  |
Equity, beginning balance |  |  | 829.6 |  |
Equity, ending balance | 833.5 |  | 833.5 |  |
Defined benefit plan and other postretirement liabilities [Member] |  |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |  |
Other comprehensive loss before income tax effect | (97.7) |  | (97.7) |  | (102.1)
Income tax effect | 7.0 |  | 7.0 |  | 8.2
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |  |
Equity, beginning balance | (90.8) | $ (115.2) | (93.8) | $ (116.2) |
Net amount reclassified to net income (loss) | 0.1 | 1.0 | 3.2 | 2.0 |
Equity, ending balance | (90.7) | (114.2) | (90.7) | (114.2) |
Unrealized gain (loss) on financial instruments [Member] |  |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |  |
Other comprehensive loss before income tax effect | 2.5 |  | 2.5 |  | 2.7
Income tax effect | (0.5) |  | (0.5) |  | $ (0.5)
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |  |
Equity, beginning balance | 2.1 | 2.2 | 2.1 | 2.3 |
Net amount reclassified to net income (loss) | 0.0 | 0.0 | (0.1) | (0.1) |
Equity, ending balance | 2.1 | 2.2 | 2.1 | 2.2 |
AOCI Attributable to Parent [Member] |  |  |  |  |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |  |
Equity, beginning balance | (88.7) | (113.0) | (91.7) | (113.9) |
Net amount reclassified to net income (loss) | 0.1 | 1.0 | 3.1 | 1.9 |
Equity, ending balance | $ (88.7) | $ (112.0) | $ (88.7) | $ (112.0) |

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