Derivative Instruments and Hedging Activities (Oil and Natural Gas Commodity Derivatives Outstanding) (Details)	Sep. 30, 2012
Year 2012 [Member] | Crude Oil Contracts [Member] | Q4 [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Volume per Day	54,250
Year 2012 [Member] | Collars [Member] | Crude Oil Contracts [Member] | Q4 [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Swap price	0
Floor price	80.00
Ceiling price	128.57
Volume per Day	53,000
Year 2012 [Member] | Collars [Member] | Crude Oil Contracts [Member] | Q4 [Member] | Maximum [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Derivative Price Range	140.65
Year 2012 [Member] | Collars [Member] | Crude Oil Contracts [Member] | Q4 [Member] | Minimum [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Derivative Price Range	80.00
Year 2012 [Member] | Swap [Member] | Crude Oil Contracts [Member] | Q4 [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Swap price	81.04
Floor price	0
Ceiling price	0
Volume per Day	625
Year 2012 [Member] | Swap [Member] | Crude Oil Contracts [Member] | Q4 [Member] | Maximum [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Derivative Price Range	81.75
Year 2012 [Member] | Swap [Member] | Crude Oil Contracts [Member] | Q4 [Member] | Minimum [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Derivative Price Range	80.28
Year 2012 [Member] | Swap [Member] | Natural Gas Contracts [Member] | Q4 [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Swap price	6.53
Floor price	0
Ceiling price	0
Volume per Day	20,000
Year 2012 [Member] | Swap [Member] | Natural Gas Contracts [Member] | Q4 [Member] | Maximum [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Derivative Price Range	6.85
Year 2012 [Member] | Swap [Member] | Natural Gas Contracts [Member] | Q4 [Member] | Minimum [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Derivative Price Range	6.30
Year 2012 [Member] | Put Option [Member] | Crude Oil Contracts [Member] | Q4 [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Swap price	0
Floor price	65.00
Ceiling price	0
Volume per Day	625
Year 2012 [Member] | Put Option [Member] | Crude Oil Contracts [Member] | Q4 [Member] | Maximum [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Derivative Price Range	65.00
Year 2012 [Member] | Put Option [Member] | Crude Oil Contracts [Member] | Q4 [Member] | Minimum [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Derivative Price Range	65.00
Year 2013 [Member] | Collars [Member] | Crude Oil Contracts [Member] | Q1 [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Swap price	0
Floor price	78.91
Ceiling price	108.01
Volume per Day	55,000
Year 2013 [Member] | Collars [Member] | Crude Oil Contracts [Member] | Q1 [Member] | Maximum [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Derivative Price Range	113.00
Year 2013 [Member] | Collars [Member] | Crude Oil Contracts [Member] | Q1 [Member] | Minimum [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Derivative Price Range	70.00
Year 2013 [Member] | Collars [Member] | Crude Oil Contracts [Member] | Q2 [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Swap price	0
Floor price	79.64
Ceiling price	108.61
Volume per Day	56,000
Year 2013 [Member] | Collars [Member] | Crude Oil Contracts [Member] | Q2 [Member] | Maximum [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Derivative Price Range	121.50
Year 2013 [Member] | Collars [Member] | Crude Oil Contracts [Member] | Q2 [Member] | Minimum [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Derivative Price Range	75.00
Year 2013 [Member] | Collars [Member] | Crude Oil Contracts [Member] | Q3 [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Swap price	0
Floor price	79.64
Ceiling price	109.15
Volume per Day	56,000
Year 2013 [Member] | Collars [Member] | Crude Oil Contracts [Member] | Q3 [Member] | Maximum [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Derivative Price Range	133.10
Year 2013 [Member] | Collars [Member] | Crude Oil Contracts [Member] | Q3 [Member] | Minimum [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Derivative Price Range	75.00
Year 2013 [Member] | Collars [Member] | Crude Oil Contracts [Member] | Q4 [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Swap price	0
Floor price	80.00
Ceiling price	117.53
Volume per Day	54,000
Year 2013 [Member] | Collars [Member] | Crude Oil Contracts [Member] | Q4 [Member] | Maximum [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Derivative Price Range	127.50
Year 2013 [Member] | Collars [Member] | Crude Oil Contracts [Member] | Q4 [Member] | Minimum [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Derivative Price Range	80.00
Year 2014 [Member] | Collars [Member] | Crude Oil Contracts [Member] | Q1 [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Swap price	0
Floor price	80.00
Ceiling price	103.13
Volume per Day	46,000
Year 2014 [Member] | Collars [Member] | Crude Oil Contracts [Member] | Q1 [Member] | Maximum [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Derivative Price Range	104.5
Year 2014 [Member] | Collars [Member] | Crude Oil Contracts [Member] | Q1 [Member] | Minimum [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Derivative Price Range	80
Year 2014 [Member] | Collars [Member] | Crude Oil Contracts [Member] | Q2 [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Swap price	0
Floor price	80.00
Ceiling price	103.13
Volume per Day	46,000
Year 2014 [Member] | Collars [Member] | Crude Oil Contracts [Member] | Q2 [Member] | Maximum [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Derivative Price Range	104.5
Year 2014 [Member] | Collars [Member] | Crude Oil Contracts [Member] | Q2 [Member] | Minimum [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Derivative Price Range	80