Derivative Instruments - Estimated Fair Value of Derivative Instruments (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Derivatives, Fair Value [Line Items]
Derivative assets, gross	$ 47,152		$ 40,252
Current derivative assets, net	35,310		12,840
Long-term derivative assets, net	7,208		16,785
Derivative liabilities, gross	(6,433)		(12,259)
Current derivative liabilities, net	(536)		(1,505)
Long-term derivative liabilities, net	(1,263)		(127)
Derivative assets (liabilities), net	40,719		27,993
Current derivative assets (liabilities), net	34,774		11,335
Long-term derivative assets (liabilities), net	5,945		16,658
Oil Derivative | Swaps
Derivatives, Fair Value [Line Items]
Derivative assets, gross	5,980		3,832
Derivative liabilities, gross	(380)		(9,744)
Derivative assets (liabilities), net	5,600		(5,912)
Oil Derivative | Costless Collars
Derivatives, Fair Value [Line Items]
Derivative assets, gross	25,529		6,296
Derivative liabilities, gross	0		(1,247)
Derivative assets (liabilities), net	25,529		5,049
Natural Gas Derivative | Swaps
Derivatives, Fair Value [Line Items]
Derivative assets, gross	15,614		30,124
Derivative liabilities, gross	(4,453)		0
Derivative assets (liabilities), net	11,161		30,124
Natural Gas Derivative | Basis Protection Swaps
Derivatives, Fair Value [Line Items]
Derivative assets, gross	29		0
Derivative liabilities, gross	(1,600)		(1,268)
Derivative assets (liabilities), net	(1,571)		(1,268)
Netting adjustments
Derivatives, Fair Value [Line Items]
Derivative assets, gross	(4,634)	[1]	(10,627)	[1]
Derivative liabilities, gross	$ 4,634	[1]	$ 10,627	[1]

[1] Amounts represent the impact of master netting agreements that allow us to net settle positive and negative positions with the same counterparty.