Derivative Instruments - Outstanding Derivatives (Details) (USD $) In Thousands, unless otherwise specified								3 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2013 Swaps Derivatives Maturing In 2013 Oil Derivative bbl	Jun. 30, 2013 Swaps Derivatives Maturing In 2013 Natural Gas Derivative MMBTU	Jun. 30, 2013 Swaps Derivatives Maturing In 2014 Oil Derivative bbl	Jun. 30, 2013 Swaps Derivatives Maturing In 2014 Natural Gas Derivative MMBTU	Jun. 30, 2013 Swaps Derivatives maturing in 2015 Natural Gas Derivative MMBTU	Jun. 30, 2013 Put Options Purchased Oil Derivative	Jun. 30, 2013 Put Options Purchased Derivatives Maturing In 2014 Oil Derivative bbl	Jun. 30, 2013 Enhanced Swap Derivatives Maturing In 2014 Oil Derivative bbl	Jun. 30, 2013 Enhanced Swap Derivatives maturing in 2015 Oil Derivative bbl	Jun. 30, 2013 Put Spread Enhanced Swap Derivatives Maturing In 2014 Oil Derivative bbl	Jun. 30, 2013 Costless Collars Derivatives Maturing In 2013 Oil Derivative bbl	Jun. 30, 2013 Costless Collars Derivatives Maturing In 2014 Oil Derivative bbl	Jun. 30, 2013 Basis Protection Swaps Derivatives Maturing In 2013 Natural Gas Derivative MMBTU	Jun. 30, 2013 Basis Protection Swaps Derivatives Maturing In 2014 Natural Gas Derivative MMBTU
Derivative [Line Items]
Volume of derivative instruments held			838,000	10,080,000	480,000	18,420,000	10,800,000		840,000	1,440,000	4,918,000	1,303,000	1,830,000	2,400,000	8,100,000	14,090,000
Weighted average fixed price per unit, swaps			95.37	4.25	93.73	4.03	4.26			96.30	93.35	93.74			0.20	0.23
Weighted average fixed price per Bbl, sold puts										80.00	80.00	80.00	77.79	75.50
Weighted average fixed price per Bbl, purchased puts									60.00			60.00	99.87	93.25
Weighted average fixed price per Bbl, sold calls													114.30	101.94
Premium paid for put options on commodity derivative contracts								$ 664
Derivative settlements	4,513	8,013
Derivative settlements payable	338	41
Cash flow hedge gains deferred through AOCI [Abstract]
Accumulated other comprehensive income, before tax	17,622	37,134
Accumulated other comprehensive income, net of taxes	$ 11,171	$ 23,223