Components of accumulated other comprehensive loss (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive loss before income tax effect	$ (119,985)		$ (119,985)		$ (123,057)
Income tax effect	8,020		8,020		9,164
Accumulated other comprehensive loss	(111,965)		(111,965)		(113,893)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Equity, beginning balance			756,655
Equity, ending balance	773,094		773,094
Defined benefit plan and other postretirement liabilities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive loss before income tax effect	(122,705)		(122,705)		(125,917)
Income tax effect	8,564		8,564		9,736
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Equity, beginning balance	(115,225)	$ (108,944)	(116,181)	$ (110,667)
Net amount reclassified to net income	1,084	862	2,040	2,585
Equity, ending balance	(114,141)	(108,082)	(114,141)	(108,082)
Unrealized gain (loss) on financial instruments [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive loss before income tax effect	2,720		2,720		2,860
Income tax effect	(544)		(544)		$ (572)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Equity, beginning balance	2,213	(2,059)	2,288	(1,983)
Net amount reclassified to net income	(37)	(38)	(112)	(114)
Equity, ending balance	2,176	(2,097)	2,176	(2,097)
AOCI Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Equity, beginning balance	(113,012)	(111,003)	(113,893)	(112,650)
Net amount reclassified to net income	1,047	824	1,928	2,471
Equity, ending balance	$ (111,965)	$ (110,179)	$ (111,965)	$ (110,179)