Gains and losses on derivative instruments (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
Derivative instruments [Line Items]
Amount of gain (loss) recognized in accumulated other comprehensive loss (effective portion), net of tax	$ 0	$ 254	$ 0	$ (5,594)
Amount of (gain) loss reclassified from accumulated other comprehensive loss (effective portion), net of tax	13	(395)	357	(3,168)
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	0	0	(6,154)
Amount of (gain) loss reclassified from accumulated other comprehensive loss (effective portion), net of tax	(86)	(871)	158	(3,714)
Amount of gain (loss) recognized in income statement (ineffective portion), before tax	0	0	0	(1,422)
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	0	254	0	560
Amount of (gain) loss reclassified from accumulated other comprehensive loss (effective portion), net of tax	99	475	199	546
Amount of gain (loss) recognized in income statement (ineffective portion), before tax	0	(610)	0	(769)
Commodity derivatives not designated as hedging instruments:
Derivative instruments [Line Items]
Amount of gain (loss) recognized in operating revenues, before tax	$ (5,196)	$ 13,047	$ (11,908)	$ 8,637