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Components of accumulated other comprehensive loss (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Other comprehensive loss before income tax effect $ (98.6)   $ (99.4)
Income tax effect 7.3   7.7
Accumulated other comprehensive loss (91.3)   (91.7)
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Equity, beginning balance 829.6    
Equity, ending balance 830.4    
Defined benefit plan and other postretirement liabilities [Member]      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Other comprehensive loss before income tax effect (101.2)   (102.1)
Income tax effect 7.8   8.2
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Equity, beginning balance (93.8) $ (116.2)  
Net amount reclassified to net loss 0.4 0.7  
Equity, ending balance (93.4) (115.5)  
Unrealized gain (loss) on financial instruments [Member]      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Other comprehensive loss before income tax effect 2.6   2.7
Income tax effect (0.5)   $ (0.5)
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Equity, beginning balance 2.1 2.3  
Net amount reclassified to net loss 0.0 0.0  
Equity, ending balance 2.1 2.3  
AOCI Attributable to Parent [Member]      
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Equity, beginning balance (91.7) (113.9)  
Net amount reclassified to net loss 0.4 0.7  
Equity, ending balance $ (91.3) $ (113.2)