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Components of accumulated other comprehensive loss (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Other comprehensive loss before income tax effect $ (122,139)   $ (123,345)
Income tax effect 10,312   10,695
Accumulated other comprehensive loss (111,827)   (112,650)
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Equity, beginning balance 1,017,870    
Net amount reclassified to net income (loss) 823 $ 78  
Equity, ending balance 1,002,851    
Defined benefit plan and other postretirement liabilities [Member]      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Other comprehensive loss before income tax effect (120,658)   (121,910)
Income tax effect 10,852   11,243
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Equity, beginning balance (110,667) (115,852)  
Net amount reclassified to net income (loss) 861 117  
Equity, ending balance (109,806) (115,735)  
Unrealized loss on financial instruments [Member]      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Other comprehensive loss before income tax effect (1,481)   (1,435)
Income tax effect (540)   $ (548)
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Equity, beginning balance (1,983) (1,830)  
Net amount reclassified to net income (loss) (38) (39)  
Equity, ending balance (2,021) (1,869)  
AOCI Attributable to Parent [Member]      
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Equity, beginning balance (112,650) (117,682)  
Equity, ending balance $ (111,827) $ (117,604)