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		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 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	$ 29	$ 69	$ 40	$ (10,882)
Gain (Loss) Recognized in other comprehensive income on derivatives	18	42	25	(6,669)
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)	5,992	8,455	15,781	29,391
Gain (Loss) recognized in income on derivatives (ineffective portion and amount excluded from effectiveness testing)	$ 0	$ (22)	$ 0	$ 835