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Components of accumulated other comprehensive loss (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]          
Other comprehensive loss before income tax effect $ (95.6)   $ (95.6)   $ (99.4)
Income tax effect 6.9   6.9   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 853.1   853.1    
Defined benefit plan and other postretirement liabilities [Member]          
Accumulated Other Comprehensive Income (Loss) [Line Items]          
Other comprehensive loss before income tax effect (98.2)   (98.2)   (102.1)
Income tax effect 7.4   7.4   8.2
AOCI Attributable to Parent, Net of Tax [Roll Forward]          
Equity, beginning balance (93.4) $ (115.5) (93.8) $ (116.2)  
Net amount reclassified to net loss 2.6 0.2 3.1 1.0  
Equity, ending balance (90.8) (115.2) (90.8) (115.2)  
Unrealized gain (loss) on financial instruments [Member]          
Accumulated Other Comprehensive Income (Loss) [Line Items]          
Other comprehensive loss before income tax effect 2.6   2.6   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 2.3 2.1 2.3  
Net amount reclassified to net 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 (91.3) (113.2) (91.7) (113.9)  
Net amount reclassified to net loss 2.6 0.2 3.0 0.9  
Equity, ending balance $ (88.7) $ (113.0) $ (88.7) $ (113.0)