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Components of accumulated other comprehensive loss (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]          
Other comprehensive loss before income tax effect $ (119,727)   $ (119,727)   $ (123,345)
Income tax effect 9,548   9,548   10,695
Accumulated other comprehensive loss (110,179)   (110,179)   (112,650)
AOCI Attributable to Parent, Net of Tax [Roll Forward]          
Equity, beginning balance     1,017,870    
Equity, ending balance 937,524   937,524    
Defined benefit plan and other postretirement liabilities [Member]          
Accumulated Other Comprehensive Income (Loss) [Line Items]          
Other comprehensive loss before income tax effect (118,153)   (118,153)   (121,910)
Income tax effect 10,071   10,071   11,243
AOCI Attributable to Parent, Net of Tax [Roll Forward]          
Equity, beginning balance (108,944) $ (96,335) (110,667) $ (115,852)  
Other comprehensive income before reclassifications (1)   (4,114)   13,928  
Net amount reclassified to net income (loss) 862 529 2,585 2,004  
Equity, ending balance (108,082) (99,920) (108,082) (99,920)  
Unrealized loss on financial instruments [Member]          
Accumulated Other Comprehensive Income (Loss) [Line Items]          
Other comprehensive loss before income tax effect (1,574)   (1,574)   (1,435)
Income tax effect (523)   (523)   $ (548)
AOCI Attributable to Parent, Net of Tax [Roll Forward]          
Equity, beginning balance (2,059) (1,907) (1,983) (1,830)  
Other comprehensive income before reclassifications (1)   0   0  
Net amount reclassified to net income (loss) (38) (39) (114) (116)  
Equity, ending balance (2,097) (1,946) (2,097) (1,946)  
AOCI Attributable to Parent [Member]          
AOCI Attributable to Parent, Net of Tax [Roll Forward]          
Equity, beginning balance (111,003) (98,242) (112,650) (117,682)  
Other comprehensive income before reclassifications (1)   (4,114)   13,928  
Net amount reclassified to net income (loss) 824 490 2,471 1,888  
Equity, ending balance $ (110,179) $ (101,866) $ (110,179) $ (101,866)