Gains and losses on derivative instruments (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Derivative instruments [Line Items]
Amount of (gain) loss reclassified from accumulated other comprehensive loss (effective portion), net of tax	$ 99	$ 344
Designated as Hedging Instrument [Member] | Commodity derivatives designated as cash flow hedges [Member]
Derivative instruments [Line Items]
Amount of (gain) loss reclassified from accumulated other comprehensive loss (effective portion), net of tax	0	244
Designated as Hedging Instrument [Member] | Interest rate derivatives designated as cash flow hedges [Member]
Derivative instruments [Line Items]
Amount of (gain) loss reclassified from accumulated other comprehensive loss (effective portion), net of tax	99	100
Not designated as hedging instrument [Member] | Commodity derivatives [Member]
Derivative instruments [Line Items]
Amount of gain (loss) recognized in operating revenues, before tax	$ (11,208)	$ (6,712)