Derivative Commodity Instruments Derivatives commodity instruments - Cash flow hedging relationship in financial statements (Details) (Commodity contracts, 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 and Hedging Activities Disclosures [Line Items]
Net gain (loss) recognized in other comprehensive income on derivatives (effective portion), net of tax of $7 and ($6,712)	$ 9	$ 15,847	$ 11	$ (10,951)
Current period change in fair value of interest rate swap, tax	6	9,713	7	(6,712)
Gain (loss) on derivative instruments, net
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Gain reclassified from accumulated other comprehensive income into income (effective portion)	5,735	3,112	9,789	20,935
Gain recognized in income on derivatives (ineffective portion and amount excluded from effectiveness testing)	$ 0	$ 1,392	$ 0	$ 858