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Components of accumulated other comprehensive loss (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Other comprehensive loss before income tax effect $ (121,271)   $ (123,057)
Income tax effect 8,090   9,164
Accumulated other comprehensive loss (113,181)   (113,893)
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Equity, beginning balance 756,655    
Equity, ending balance 742,744    
Defined benefit plan and other postretirement liabilities [Member]      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Other comprehensive loss before income tax effect (124,085)   (125,917)
Income tax effect 8,654   9,736
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Equity, beginning balance (116,181) $ (110,667)  
Net amount reclassified to net loss 750 861  
Equity, ending balance (115,431) (109,806)  
Unrealized loss on financial instruments [Member]      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Other comprehensive loss before income tax effect 2,814   2,860
Income tax effect (564)   $ (572)
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Equity, beginning balance 2,288 (1,983)  
Net amount reclassified to net loss (38) (38)  
Equity, ending balance 2,250 (2,021)  
AOCI Attributable to Parent [Member]      
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Equity, beginning balance (113,893) (112,650)  
Net amount reclassified to net loss 712 823  
Equity, ending balance $ (113,181) $ (111,827)