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Commitments and contingencies - Legal Contingencies (Details) - USD ($)
 12 Months Ended 45 Months Ended
Feb. 09, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2016
Loss Contingencies [Line Items]            
Estimated litigation liability   $ 23,000,000       $ 23,000,000
Pension contributions   0 $ 34,595,000 $ 0    
PBGC Agreement [Member]            
Loss Contingencies [Line Items]            
Required pension contributions above minimum           $ 17,400,000
Pension contributions   0 $ 1,100,000 $ 0 $ 6,700,000  
Pension contributions above minimum remaining   $ 9,600,000        
Subsequent Event [Member]            
Loss Contingencies [Line Items]            
Litigation settlement amount $ 23,000,000          
Litigation settlement period 10 years          
Litigation settlement payable upon final approval of settlement $ 5,000,000          
Litigation settlement, annual installment $ 2,000,000          
Litigation settlement installment period 9 years