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Commitments and contingencies - Legal Contingencies (Details) - USD ($)
 1 Months Ended 12 Months Ended
Aug. 18, 2017
Feb. 09, 2017
Sep. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Loss Contingencies [Line Items]                
Gain recognized       $ 5,500,000 $ (3,830,000) $ (30,850,000)    
Accrued and other current liabilities       61,398,000 50,100,000      
Pension contributions       0 0 34,595,000    
Ravenswood Retiree Medical Benefits Changes [Member]                
Loss Contingencies [Line Items]                
Litigation settlement amount $ 23,000,000              
Litigation settlement period 10 years              
Ravenswood retiree legal settlement     $ 5,000,000          
Gain recognized       5,500,000        
Loss contingency accrual       12,500,000        
Accrued and other current liabilities       $ 12,856,000        
Litigation settlement, annual installment   $ 2,000,000            
Litigation settlement installment period       9 years        
PBGC Agreement [Member]                
Loss Contingencies [Line Items]                
Required pension contributions above minimum               $ 17,400,000
Pension contributions       $ 0 $ 0 $ 1,100,000 $ 0 $ 6,700,000
Pension contributions above minimum remaining       $ 9,600,000