Risk Management and Derivative Instruments (Details) (Not designated as Hedging Instrument, Commodity Derivatives, USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2013 bbl
Risk Management and Derivative Instruments
Maximum loss exposure under commodity derivative contracts on failure of the entity's counterparty's performance	$ 13.6
Oil | WTI | Swaps | 2013
Risk Management and Derivative Instruments
Hedged Volume	2,172,214
Weighted-Average Fixed Price	94.12
Oil | WTI | Swaps | 2014
Risk Management and Derivative Instruments
Hedged Volume	4,344,450
Weighted-Average Fixed Price	88.76
Oil | WTI | Swaps | 2015
Risk Management and Derivative Instruments
Hedged Volume	1,820,000
Weighted-Average Fixed Price	86.55
Oil | WTI | Collars | 2013
Risk Management and Derivative Instruments
Hedged Volume	101,502
Oil | WTI | Collars | 2013 | Minimum
Risk Management and Derivative Instruments
Weighted-Average Fixed Price	85.27
Oil | WTI | Collars | 2013 | Maximum
Risk Management and Derivative Instruments
Weighted-Average Fixed Price	100.70
Oil | WTI | Collars | 2014
Risk Management and Derivative Instruments
Hedged Volume	164,400
Oil | WTI | Collars | 2014 | Minimum
Risk Management and Derivative Instruments
Weighted-Average Fixed Price	88.49
Oil | WTI | Collars | 2014 | Maximum
Risk Management and Derivative Instruments
Weighted-Average Fixed Price	97.94
Oil | WTI to LLS | Basis Differential Swaps | 2013
Risk Management and Derivative Instruments
Hedged Volume	743,794
Weighted-Average Fixed Price	5.78
Oil | WTI to LLS | Basis Differential Swaps | 2014
Risk Management and Derivative Instruments
Hedged Volume	501,000
Weighted-Average Fixed Price	5.35
Natural Gas | Swaps | 2013
Risk Management and Derivative Instruments
Hedged Volume	7,360,000
Weighted-Average Fixed Price	4.09
Hedged Volume, yet to be cash settled	1,240,000
Natural Gas | Swaps | 2014
Risk Management and Derivative Instruments
Hedged Volume	9,125,000
Weighted-Average Fixed Price	4.23
Natural Gas | Collars | 2013
Risk Management and Derivative Instruments
Hedged Volume	1,116,498
Natural Gas | Collars | 2013 | Minimum
Risk Management and Derivative Instruments
Weighted-Average Fixed Price	3.68
Natural Gas | Collars | 2013 | Maximum
Risk Management and Derivative Instruments
Weighted-Average Fixed Price	4.91
Natural Gas | Collars | 2014
Risk Management and Derivative Instruments
Hedged Volume	1,685,004
Natural Gas | Collars | 2014 | Minimum
Risk Management and Derivative Instruments
Weighted-Average Fixed Price	3.99
Natural Gas | Collars | 2014 | Maximum
Risk Management and Derivative Instruments
Weighted-Average Fixed Price	5.09
NGL | Swaps | 2013
Risk Management and Derivative Instruments
Hedged Volume	129,000
Weighted-Average Fixed Price	63.42
NGL | Swaps | 2014
Risk Management and Derivative Instruments
Hedged Volume	151,500
Weighted-Average Fixed Price	62.16