Gains and losses on derivatives instruments (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
Derivative instruments [Line Items]
Amount of gain (loss) recognized in accumulated other comprehensive loss (effective portion), net of tax	$ 254	$ 25,773	$ (5,594)	$ 22,506
Amount of (gain) loss reclassified from accumulated other comprehensive (effective portion), net of tax	(396)	(1,834)	(3,168)	(4,666)
Commodity derivatives designated as cash flow hedges [Member]
Derivative instruments [Line Items]
Amount of gain (loss) recognized in accumulated other comprehensive loss (effective portion), net of tax	0	28,018	(6,154)	23,863
Amount of (gain) loss reclassified from accumulated other comprehensive (effective portion), net of tax	(871)	(1,840)	(3,714)	(4,687)
Amount of gain (loss) recognized in income statement (ineffective portion), before tax	0	3,863	(1,422)	(388)
Interest rate derivatives designated as cash flow hedges [Member]
Derivative instruments [Line Items]
Amount of gain (loss) recognized in accumulated other comprehensive loss (effective portion), net of tax	254	(2,245)	560	(1,357)
Amount of (gain) loss reclassified from accumulated other comprehensive (effective portion), net of tax	475	6	546	21
Amount of gain (loss) recognized in income statement (ineffective portion), before tax	(610)	0	(769)	0
Commodity derivatives not designated as hedging instruments:
Derivative instruments [Line Items]
Amount of gain (loss) recognized in operating revenues, before tax	$ 13,047	$ 993	$ 8,637	$ 1,048