Reclassifications out of accumulated other comprehensive (loss) (Details 13) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest expense	$ (82,788)	$ (87,848)	$ (91,179)
Income taxes	(65,041)	(93,132)	(70,664)
Other income	4,103	4,956	18,457
Net income (loss)	281,203	(67,258)	$ (657,691)
Amounts reclassified from accumulated other comprehensive loss	(1,499)	(3,016)
Reclassification adjustment for loss on derivative instruments included in net income (loss):
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from accumulated other comprehensive loss	366	367
Reclassification adjustment for loss on derivative instruments included in net income (loss): | Reclassification out of accumulated other comprehensive income [Member] | Interest rate contract [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest expense	(590)	(593)
Income taxes	224	226
Net income (loss)	(366)	(367)
Amortization of postretirement liability losses included in net periodic benefit cost (credit)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from accumulated other comprehensive loss	1,013	2,506
Amortization of postretirement liability losses included in net periodic benefit cost (credit) | Reclassification out of accumulated other comprehensive income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income taxes	645	1,425
Net periodic benefit cost	(1,658)	(3,931)
Net income (loss)	(1,013)	(2,506)
Reclassification adjustment for loss on foreign currency translation adjustment included in net income (loss)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from accumulated other comprehensive loss	0	0
Reclassification adjustment for loss on available-for-sale investments included in net income (loss)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from accumulated other comprehensive loss	120	143
Reclassification adjustment for loss on available-for-sale investments included in net income (loss) | Reclassification out of accumulated other comprehensive income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income taxes	65	77
Other income	(185)	(220)
Net income (loss)	$ (120)	$ (143)