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Business and summary of significant accounting policies (Narrative) (Details)
$ in Millions
 5 Months Ended 7 Months Ended 12 Months Ended
Dec. 31, 2015
USD ($)
Aug. 14, 2015
USD ($)
Dec. 31, 2017
USD ($)
category
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Business And Summary Of Significant Accounting Policies [Line Items]          
Number of major product categories | category     2    
Depreciation expense $ 18.8 $ 26.7 $ 50.4 $ 63.4  
Capital expenditures within accounts payable     13.6    
Bank fees capitalized     0.4 0.7  
Amortization of capitalized bank fees 0.0 2.1 0.3 0.2  
Environmental remediation, compliance and management expenses     8.0 8.3 $ 6.5
Accrued liability relating to environmental remediation     2.1 5.2  
Deferred costs     0.0 0.0  
Amortized costs $ 2.1 $ 4.3 $ 3.3 $ 7.0  
Commodity derivative contracts | Minimum          
Business And Summary Of Significant Accounting Policies [Line Items]          
Derivative maturities     3 years    
Commodity derivative contracts | Maximum          
Business And Summary Of Significant Accounting Policies [Line Items]          
Derivative maturities     5 years