Reclassifications out of accumulated other comprehensive (loss) (Details 14) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest expense	$ (87,848)	$ (91,179)	$ (86,871)
Income taxes	(93,132)	(70,664)	(64,422)
Other income	4,956	18,457	9,138
Net income (loss)	(67,258)	(657,691)	$ 294,338
Reclassification from accumulated other comprehensive income, current period, net of tax	(3,016)	(3,131)
Reclassification adjustment for loss on derivative instruments included in net income (loss):
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification from accumulated other comprehensive income, current period, net of tax	367	404
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	(593)	(637)
Income taxes	226	233
Net income (loss)	(367)	(404)
Amortization of postretirement liability losses included in net periodic benefit cost (credit)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification from accumulated other comprehensive income, current period, net of tax	2,506	1,794
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	1,425	1,128
Net periodic benefit cost	(3,931)	(2,922)
Net income (loss)	(2,506)	(1,794)
Reclassification adjustment for loss on foreign currency translation adjustment included in net income (loss)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification from accumulated other comprehensive income, current period, net of tax	0	802
Reclassification adjustment for loss on foreign currency translation adjustment 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	0	490
Other income	0	(1,292)
Net income (loss)	0	(802)
Reclassification adjustment for loss on available-for-sale investments included in net income (loss)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification from accumulated other comprehensive income, current period, net of tax	143	131
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	77	70
Other income	(220)	(201)
Net income (loss)	$ (143)	$ (131)