Reclassification out of accumulated other comprehensive income (loss) (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Revenues	$ 1,094,046	$ 1,060,595	$ 2,136,899	$ 1,992,198
Interest expense	(21,516)	(21,427)	(42,487)	(42,300)
Other income	2,777	1,436	4,908	2,677
Income before income taxes	79,898	71,380	164,014	155,967
Income taxes	27,118	24,988	55,050	52,983
Net income (loss)	53,327	46,333	109,466	102,848
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	(23)	717	(570)	5,116
Income taxes	10	(322)	213	(1,948)
Net income (loss)	(13)	395	(357)	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	(395)	(967)	(838)	(1,934)
Income taxes	150	543	318	862
Net income (loss)	(245)	(424)	(520)	(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	(49)	(66)	(49)	(121)
Income taxes	17	23	17	42
Net income (loss)	(32)	(43)	(32)	(79)
Total reclassifications
Reclassification adjustment out of accumulated other comprehensive income [Line Items]
Net income (loss)	(290)	(72)	(909)	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	137	1,381	(250)	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	$ (160)	$ (664)	$ (320)	$ (780)