Reclassification out of accumulated other comprehensive income (loss) (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Operating revenues	$ 918,510	$ 1,042,853
Interest expense	(23,149)	(20,971)
Income before income taxes	(495,172)	84,116
Other income	2,324	2,183
Income taxes	185,727	(27,932)
Net income (loss)	(309,445)	56,139
Reclassification adjustment for gain (loss) on derivative instruments included in net income
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Income before income taxes	(159)	(548)
Income taxes	60	204
Net income (loss)	(99)	(344)
Amortization of postretirement liability losses included in net periodic benefit cost
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Net periodic benefit cost	(605)	(443)
Income taxes	230	168
Net income (loss)	(375)	(275)
Reclassification adjustment for loss on foreign currency translation adjustment included in net income
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Other income	(1,292)	0
Income taxes	490	0
Net income (loss)	(802)	0
Reclassification adjustment for loss on available-for-sale investments included in net income
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Other income	(55)	(58)
Income taxes	19	20
Net income (loss)	(36)	(38)
Total reclassifications
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Net income (loss)	(1,312)	(657)
Commodity derivative instruments [Member] | Reclassification adjustment for gain (loss) on derivative instruments included in net income
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Operating revenues	0	(388)
Interest rate derivative instruments [Member] | Reclassification adjustment for gain (loss) on derivative instruments included in net income
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Interest expense	$ (159)	$ (160)