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Goodwill and other intangible assets (Narrative) (Details) - USD ($)
$ in Millions
 3 Months Ended 5 Months Ended 6 Months Ended 7 Months Ended 12 Months Ended
Jun. 30, 2015
Dec. 31, 2015
Jun. 30, 2018
Jun. 30, 2017
Aug. 14, 2015
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Mar. 31, 2018
Goodwill [Line Items]                  
Amortization expense of intangible assets   $ 5.5 $ 6.6 $ 6.9   $ 13.6 $ 14.3    
Future Amortization Expense, 2018           12.9      
Future Amortization Expense, 2019           12.2     $ 12.2
Future Amortization Expense, 2020           11.4     11.4
Future Amortization Expense, 2021           10.7     10.7
Future Amortization Expense, 2022           $ 10.1     $ 10.1
Predecessor [Member]                  
Goodwill [Line Items]                  
Amortization expense of intangible assets         $ 10.5        
Needle Coke [Member]                  
Goodwill [Line Items]                  
Percentage Decrease in Market Prices 18.00%                
Fair value inputs, Discount rate           10.50%      
Goodwill, Impairment Loss               $ 35.4