Reclassification out of accumulated other comprehensive income (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Revenues	$ 1,060,595	$ 967,962	$ 1,992,198	$ 1,820,769
Interest expense	21,427	17,650	42,300	37,089
Income before income taxes	71,380	75,698	155,967	129,667
Income taxes	24,988	26,691	52,983	44,769
Net income (loss)	46,333	54,113	102,848	89,904
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	718		5,116
Income taxes	(322)		(1,948)
Net income (loss)	396		3,168
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	(967)		(1,934)
Income taxes	543		862
Net income (loss)	(424)		(1,072)
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	(67)		(121)
Income taxes	23		42
Net income (loss)	(44)		(79)
Total reclassifications
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Net income (loss)	(72)		2,017
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	1,382		5,896
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	$ (664)		$ (780)