Derivative Instruments and Hedging Activities (Commodity Derivatives Outstanding) (Details)	Mar. 31, 2013
Year 2013 [Member] | Q2 [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Volume per Day	56,000
Floor price	79.64
Ceiling price	108.61
Year 2013 [Member] | Q2 [Member] | Maximum [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Derivative Price Range	121.50
Year 2013 [Member] | Q2 [Member] | Minimum [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Derivative Price Range	75.00
Year 2013 [Member] | Q3 [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Volume per Day	56,000
Floor price	79.64
Ceiling price	109.15
Year 2013 [Member] | Q3 [Member] | Maximum [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Derivative Price Range	133.10
Year 2013 [Member] | Q3 [Member] | Minimum [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Derivative Price Range	75.00
Year 2013 [Member] | Q4 [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Volume per Day	54,000
Floor price	80.00
Ceiling price	117.53
Year 2013 [Member] | Q4 [Member] | Maximum [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Derivative Price Range	127.50
Year 2013 [Member] | Q4 [Member] | Minimum [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Derivative Price Range	80.00
Year 2014 [Member] | Q1 [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Volume per Day	52,000
Floor price	80.00
Ceiling price	102.44
Year 2014 [Member] | Q1 [Member] | Maximum [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Derivative Price Range	104.50
Year 2014 [Member] | Q1 [Member] | Minimum [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Derivative Price Range	80.00
Year 2014 [Member] | Q2 [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Volume per Day	52,000
Floor price	80.00
Ceiling price	102.44
Year 2014 [Member] | Q2 [Member] | Maximum [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Derivative Price Range	104.50
Year 2014 [Member] | Q2 [Member] | Minimum [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Derivative Price Range	80.00
Year 2014 [Member] | Q3 [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Volume per Day	48,000
Floor price	80.00
Ceiling price	97.46
Year 2014 [Member] | Q3 [Member] | Maximum [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Derivative Price Range	98.80
Year 2014 [Member] | Q3 [Member] | Minimum [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Derivative Price Range	80.00
Year 2014 [Member] | Q4 [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Volume per Day	48,000
Floor price	80.00
Ceiling price	97.46
Year 2014 [Member] | Q4 [Member] | Maximum [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Derivative Price Range	98.80
Year 2014 [Member] | Q4 [Member] | Minimum [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Derivative Price Range	80.00
Year 2015 [Member] | Q1 [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Volume per Day	9,000
Floor price	80.00
Ceiling price	100.59
Year 2015 [Member] | Q1 [Member] | Maximum [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Derivative Price Range	100.90
Year 2015 [Member] | Q1 [Member] | Minimum [Member]
Commodity Derivative Contracts Not Classified as Hedging Instruments
Derivative Price Range	80.00