Derivative Instruments Derivative instruments (Details 2) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Derivative [Line Items]
Net unrealized gain (loss) on derivative instruments arising during the period, net of tax	$ 8,497	$ 7,900	$ (3,077)
Amount of gain (loss) reclassified from accumulated other comprehensive loss into operating revenues (effective portion), net of tax	8,754	0	(3,750)
Commodity derivatives designated as cash flow hedges:
Derivative [Line Items]
Net unrealized gain (loss) on derivative instruments arising during the period, net of tax	10,209	10,806	(3,077)
Amount of gain (loss) reclassified from accumulated other comprehensive loss into operating revenues (effective portion), net of tax	8,788	0	(3,720)
Amount of gain (loss) recognized in operating revenues (ineffective portion), before tax	(730)	1,827	0
Interest rate derivatives designated as cash flow hedges:
Derivative [Line Items]
Net unrealized gain (loss) on derivative instruments arising during the period, net of tax	(1,712)	(2,906)	0
Amount of gain (loss) reclassified from accumulated other comprehensive loss into operating revenues (effective portion), net of tax	(34)	0	(30)
Amount of gain (loss) recognized in operating revenues (ineffective portion), before tax	0	0	0
Commodity derivatives not designated as hedging instruments:
Derivative [Line Items]
Amount of gain recognized in operating revenues, before tax	$ 106	$ 0	$ 0