Derivative Instruments - Outstanding Derivatives (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2012	Dec. 31, 2011
Cash flow hedge gains deferred through AOCI [Abstract]
Accumulated other comprehensive income, before tax	$ 59,571	$ 83,880
Accumulated other comprehensive income, net of taxes	38,125	51,846
Cash flow hedge gain to be reclassified within twelve months	41,948
Cash flow hedge gains to be reclassified within twelve months, net of tax	26,847
Derivative settlements receivable	8,208	449
Derivative settlements payable	$ 0	$ 5,042
Swaps | Oil Derivative
Derivative [Line Items]
Volume of derivative instruments held	1,543,000
Swaps | Natural Gas Derivative
Derivative [Line Items]
Volume of derivative instruments held	25,200,000
Swaps | Derivatives Maturing In 2012 | Oil Derivative
Derivative [Line Items]
Volume of derivative instruments held	1,003,000
Weighted average fixed price per unit, swaps	94.37
Swaps | Derivatives Maturing In 2012 | Natural Gas Derivative
Derivative [Line Items]
Volume of derivative instruments held	8,400,000
Weighted average fixed price per unit, swaps	4.52
Swaps | Derivatives Maturing In 2013 | Oil Derivative
Derivative [Line Items]
Volume of derivative instruments held	540,000
Weighted average fixed price per unit, swaps	102.45
Swaps | Derivatives Maturing In 2013 | Natural Gas Derivative
Derivative [Line Items]
Volume of derivative instruments held	12,000,000
Weighted average fixed price per unit, swaps	4.52
Swaps | Derivatives Maturing In 2014 | Natural Gas Derivative
Derivative [Line Items]
Volume of derivative instruments held	4,800,000
Weighted average fixed price per unit, swaps	3.84
Costless Collars | Oil Derivative
Derivative [Line Items]
Volume of derivative instruments held	4,590,000
Costless Collars | Derivatives Maturing In 2012 | Oil Derivative
Derivative [Line Items]
Volume of derivative instruments held	1,060,000
Weighted average fixed price per Bbl, sold puts	73.87
Weighted average fixed price per Bbl, purchased puts	96.30
Weighted average fixed price per Bbl, sold calls	108.78
Costless Collars | Derivatives Maturing In 2013 | Oil Derivative
Derivative [Line Items]
Volume of derivative instruments held	3,410,000
Weighted average fixed price per Bbl, sold puts	78.14
Weighted average fixed price per Bbl, purchased puts	100.32
Weighted average fixed price per Bbl, sold calls	115.29
Costless Collars | Derivatives Maturing In 2014 | Oil Derivative
Derivative [Line Items]
Volume of derivative instruments held	120,000
Weighted average fixed price per Bbl, sold puts	85.00
Weighted average fixed price per Bbl, purchased puts	100.00
Weighted average fixed price per Bbl, sold calls	116.65
Basis Protection Swaps | Natural Gas Derivative
Derivative [Line Items]
Volume of derivative instruments held	39,490,000
Basis Protection Swaps | Derivatives Maturing In 2012 | Natural Gas Derivative
Derivative [Line Items]
Volume of derivative instruments held	9,000,000
Weighted average fixed price per unit, swaps	0.23
Basis Protection Swaps | Derivatives Maturing In 2013 | Natural Gas Derivative
Derivative [Line Items]
Volume of derivative instruments held	16,400,000
Weighted average fixed price per unit, swaps	0.20
Basis Protection Swaps | Derivatives Maturing In 2014 | Natural Gas Derivative
Derivative [Line Items]
Volume of derivative instruments held	14,090,000
Weighted average fixed price per unit, swaps	0.23