Risk Management and Derivative Instruments (Details) (Not designated as Hedging Instrument, Commodity Derivatives, USD $) In Millions, unless otherwise specified	Jun. 30, 2012
Risk Management and Derivative Instruments
Maximum loss exposure under commodity derivative contracts on failure of the entity's counterparty's performance	$ 18.3
Swaps | 2012
Risk Management and Derivative Instruments
Hedged Volume (in Bbls)	411,100
Weighted-Average Fixed Price (in dollars per Bbls)	84.36
Swaps | 2013
Risk Management and Derivative Instruments
Hedged Volume (in Bbls)	679,125
Weighted-Average Fixed Price (in dollars per Bbls)	84.73
Swaps | 2014
Risk Management and Derivative Instruments
Hedged Volume (in Bbls)	262,450
Weighted-Average Fixed Price (in dollars per Bbls)	83.00
Collars | 2012
Risk Management and Derivative Instruments
Hedged Volume (in Bbls)	82,800
Collars | 2012 | Minimum
Risk Management and Derivative Instruments
Weighted-Average Fixed Price (in dollars per Bbls)	85.00
Collars | 2012 | Maximum
Risk Management and Derivative Instruments
Weighted-Average Fixed Price (in dollars per Bbls)	127.28
Deferred Premium Puts | 2012
Risk Management and Derivative Instruments
Hedged Volume (in Bbls)	276,000
Weighted-Average Fixed Price (in dollars per Bbls)	79.01
Strike price of a put option (in dollars per Bbls)	85.00
Deferred premium (in dollars per Bbls)	5.99
Basis Differential Swaps | 2012
Risk Management and Derivative Instruments
Hedged Volume (in Bbls)	505,300
Weighted-Average Fixed Price (in dollars per Bbls)	9.73
Basis Differential Swaps | 2013
Risk Management and Derivative Instruments
Hedged Volume (in Bbls)	679,125
Weighted-Average Fixed Price (in dollars per Bbls)	6.30
LLS Swaps | 2012
Risk Management and Derivative Instruments
Hedged Volume (in Bbls)	315,180
Weighted-Average Fixed Price (in dollars per Bbls)	116.55
Brent Swaps | 2013
Risk Management and Derivative Instruments
Hedged Volume (in Bbls)	1,021,749
Weighted-Average Fixed Price (in dollars per Bbls)	111.89