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Components of accumulated other comprehensive loss (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]          
Other comprehensive loss before income tax effect $ (121,414)   $ (121,414)   $ (123,057)
Income tax effect 8,402   8,402   9,164
Accumulated other comprehensive loss (113,012)   (113,012)   (113,893)
AOCI Attributable to Parent, Net of Tax [Roll Forward]          
Equity, beginning balance     756,655    
Equity, ending balance 750,761   750,761    
Defined benefit plan and other postretirement liabilities [Member]          
Accumulated Other Comprehensive Income (Loss) [Line Items]          
Other comprehensive loss before income tax effect (124,180)   (124,180)   (125,917)
Income tax effect 8,955   8,955   9,736
AOCI Attributable to Parent, Net of Tax [Roll Forward]          
Equity, beginning balance (115,431) $ (109,806) (116,181) $ (110,667)  
Net amount reclassified to net loss 206 862 956 1,723  
Equity, ending balance (115,225) (108,944) (115,225) (108,944)  
Unrealized gain (loss) on financial instruments [Member]          
Accumulated Other Comprehensive Income (Loss) [Line Items]          
Other comprehensive loss before income tax effect 2,766   2,766   2,860
Income tax effect (553)   (553)   $ (572)
AOCI Attributable to Parent, Net of Tax [Roll Forward]          
Equity, beginning balance 2,250 (2,021) 2,288 (1,983)  
Net amount reclassified to net loss (37) (38) (75) (76)  
Equity, ending balance 2,213 (2,059) 2,213 (2,059)  
AOCI Attributable to Parent [Member]          
AOCI Attributable to Parent, Net of Tax [Roll Forward]          
Equity, beginning balance (113,181) (111,827) (113,893) (112,650)  
Net amount reclassified to net loss 169 824 881 1,647  
Equity, ending balance $ (113,012) $ (111,003) $ (113,012) $ (111,003)