Derivative Instruments - Outstanding Derivatives (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Cash flow hedge gains deferred through AOCI [Abstract]
Accumulated other comprehensive income, before tax	$ 48,103	$ 83,880
Accumulated other comprehensive income, net of taxes	30,786	51,846
Cash flow hedge gain to be reclassified within twelve months	39,513
Cash flow hedge gains to be reclassified within twelve months, net of tax	25,288
Derivative settlements receivable	5,678	449
Derivative settlements payable	$ 12	$ 5,042
Swaps | Oil Derivative
Derivative [Line Items]
Volume of derivative instruments held	1,093,000
Swaps | Natural Gas Derivative
Derivative [Line Items]
Volume of derivative instruments held	27,550,000
Swaps | Derivatives Maturing In 2012 | Oil Derivative
Derivative [Line Items]
Volume of derivative instruments held	553,000
Weighted average fixed price per unit, swaps	94.46
Swaps | Derivatives Maturing In 2012 | Natural Gas Derivative
Derivative [Line Items]
Volume of derivative instruments held	4,750,000
Weighted average fixed price per unit, swaps	4.46
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	15,600,000
Weighted average fixed price per unit, swaps	4.33
Swaps | Derivatives Maturing In 2014 | Natural Gas Derivative
Derivative [Line Items]
Volume of derivative instruments held	7,200,000
Weighted average fixed price per unit, swaps	3.91
Costless Collars | Oil Derivative
Derivative [Line Items]
Volume of derivative instruments held	4,716,000
Costless Collars | Derivatives Maturing In 2012 | Oil Derivative
Derivative [Line Items]
Volume of derivative instruments held	526,000
Weighted average fixed price per Bbl, sold puts	73.86
Weighted average fixed price per Bbl, purchased puts	96.29
Weighted average fixed price per Bbl, sold calls	108.79
Costless Collars | Derivatives Maturing In 2013 | Oil Derivative
Derivative [Line Items]
Volume of derivative instruments held	3,710,000
Weighted average fixed price per Bbl, sold puts	77.88
Weighted average fixed price per Bbl, purchased puts	99.94
Weighted average fixed price per Bbl, sold calls	114.26
Costless Collars | Derivatives Maturing In 2014 | Oil Derivative
Derivative [Line Items]
Volume of derivative instruments held	480,000
Weighted average fixed price per Bbl, sold puts	77.50
Weighted average fixed price per Bbl, purchased puts	95.24
Weighted average fixed price per Bbl, sold calls	107.84
Basis Protection Swaps | Natural Gas Derivative
Derivative [Line Items]
Volume of derivative instruments held	34,990,000
Basis Protection Swaps | Derivatives Maturing In 2012 | Natural Gas Derivative
Derivative [Line Items]
Volume of derivative instruments held	4,500,000
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