Reclassification out of accumulated other comprehensive income (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Revenues	$ 1,285,782	$ 1,173,518	$ 3,277,980	$ 2,994,288
Interest expense	21,012	19,840	63,312	56,929
Income before income taxes	131,126	(49,785)	287,093	79,882
Income taxes	46,576	(20,253)	99,559	24,516
Earnings (loss) from equity method investments	(61)	2,388	(380)	4,025
Net income (loss)	84,432	(29,671)	187,280	60,233
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	807		5,924
Income taxes	(297)		(2,246)
Net income (loss)	510		3,678
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	(437)		(2,371)
Income taxes	166		1,027
Net income (loss)	(271)		(1,344)
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	(58)		(180)
Income taxes	20		63
Net income (loss)	(38)		(117)
Reclassification adjustment for loss on foreign currency translation adjustment included in net income [Member]
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Earnings (loss) from equity method investments	(185)		(213)
Earnings (loss) from equity method investments, tax	70		70
Net income (loss)	(115)		(143)
Total reclassifications
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Net income (loss)	86		2,074
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]
Revenues	1,007		6,903
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	$ (200)		$ (979)