Derivative Commodity Instruments - Not Designated as Hedging Instruments on the Income Statement (Details) (Gain (loss) on derivative instruments, net, USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Gain (loss) on derivative instruments, net
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Open and closed non-cash mark-to-market losses on derivative instruments	$ 34,036	$ (57,446)