Derivative Instruments - Estimated Fair Value of Derivative Instruments (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012		Dec. 31, 2011
Derivatives, Fair Value [Line Items]
Derivative assets, gross	$ 79,844		$ 40,252
Current derivative assets, net	55,748		12,840
Long-term derivative assets, net	20,971		16,785
Derivative liabilities, gross	(3,390)		(12,259)
Current derivative liabilities, net	0		(1,505)
Long-term derivative liabilities, net	(265)		(127)
Derivative assets (liabilities), net	76,454		27,993
Current derivative assets (liabilities), net	55,748		11,335
Long-term derivative assets (liabilities), net	20,706		16,658
Oil Derivative | Swaps
Derivatives, Fair Value [Line Items]
Derivative assets, gross	15,471		3,832
Derivative liabilities, gross	0		(9,744)
Derivative assets (liabilities), net	15,471		(5,912)
Oil Derivative | Costless Collars
Derivatives, Fair Value [Line Items]
Derivative assets, gross	39,623		6,296
Derivative liabilities, gross	0		(1,247)
Derivative assets (liabilities), net	39,623		5,049
Natural Gas Derivative | Swaps
Derivatives, Fair Value [Line Items]
Derivative assets, gross	24,742		30,124
Derivative liabilities, gross	(1,285)		0
Derivative assets (liabilities), net	23,457		30,124
Natural Gas Derivative | Basis Protection Swaps
Derivatives, Fair Value [Line Items]
Derivative assets, gross	8		0
Derivative liabilities, gross	(2,105)		(1,268)
Derivative assets (liabilities), net	(2,097)		(1,268)
Netting adjustments
Derivatives, Fair Value [Line Items]
Derivative assets, gross	(3,125)	[1]	(10,627)	[1]
Derivative liabilities, gross	$ 3,125	[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.