Derivative Commodity Instruments - Additional Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015 counterparty	Mar. 31, 2014
Derivative [Line Items]
Number of Active Counterparties with Whom Company Holds Net Gain Positions	10
Number of Active Counterparties with Whom Company Holds Net Loss Positions	2
Gain (Loss) on fair value of derivatives	$ 34,036	$ (53,391)
Commodity contracts
Derivative [Line Items]
Gain (Loss) on fair value of derivatives	34,036	(53,391)
Commodity contracts | J.P Morgan Ventures Energy Corporation
Derivative [Line Items]
Gain (Loss) on fair value of derivatives	52,600
Commodity contracts | Merrill Lynch Commodities, Inc.
Derivative [Line Items]
Gain (Loss) on fair value of derivatives	43,500
Commodity contracts | Canadian Imperial Bank of Commerce
Derivative [Line Items]
Gain (Loss) on fair value of derivatives	33,300
Commodity contracts | Barclay Bank PLC
Derivative [Line Items]
Gain (Loss) on fair value of derivatives	28,800
Commodity contracts | Bank of Montreal
Derivative [Line Items]
Gain (Loss) on fair value of derivatives	28,700
Commodity contracts | Morgan Stanley Capital Group Inc
Derivative [Line Items]
Gain (Loss) on fair value of derivatives	$ 28,500