Derivative Instruments and Hedging Activities (Commodity Derivatives Outstanding) (Details)	Jun. 30, 2013
Year 2013 [Member] | Q3 [Member] | NYMEX [Member]
Derivative [Line Items]
Volume per Day	56,000
Floor price	79.64
Ceiling price	109.15
Year 2013 [Member] | Q3 [Member] | Minimum [Member] | NYMEX [Member]
Derivative [Line Items]
Derivative Price Range	75.00
Year 2013 [Member] | Q3 [Member] | Maximum [Member] | NYMEX [Member]
Derivative [Line Items]
Derivative Price Range	133.10
Year 2013 [Member] | Q4 [Member] | NYMEX [Member]
Derivative [Line Items]
Volume per Day	54,000
Floor price	80.00
Ceiling price	117.53
Year 2013 [Member] | Q4 [Member] | Minimum [Member] | NYMEX [Member]
Derivative [Line Items]
Derivative Price Range	80.00
Year 2013 [Member] | Q4 [Member] | Maximum [Member] | NYMEX [Member]
Derivative [Line Items]
Derivative Price Range	127.50
Year 2014 [Member] | Q1 [Member] | NYMEX [Member]
Derivative [Line Items]
Volume per Day	58,000
Floor price	80.00
Ceiling price	102.11
Year 2014 [Member] | Q1 [Member] | Minimum [Member] | NYMEX [Member]
Derivative [Line Items]
Derivative Price Range	80.00
Year 2014 [Member] | Q1 [Member] | Maximum [Member] | NYMEX [Member]
Derivative [Line Items]
Derivative Price Range	104.50
Year 2014 [Member] | Q2 [Member] | NYMEX [Member]
Derivative [Line Items]
Volume per Day	58,000
Floor price	80.00
Ceiling price	102.11
Year 2014 [Member] | Q2 [Member] | Minimum [Member] | NYMEX [Member]
Derivative [Line Items]
Derivative Price Range	80.00
Year 2014 [Member] | Q2 [Member] | Maximum [Member] | NYMEX [Member]
Derivative [Line Items]
Derivative Price Range	104.50
Year 2014 [Member] | Q3 [Member] | NYMEX [Member]
Derivative [Line Items]
Volume per Day	58,000
Floor price	80.00
Ceiling price	97.73
Year 2014 [Member] | Q3 [Member] | Minimum [Member] | NYMEX [Member]
Derivative [Line Items]
Derivative Price Range	80.00
Year 2014 [Member] | Q3 [Member] | Maximum [Member] | NYMEX [Member]
Derivative [Line Items]
Derivative Price Range	100.00
Year 2014 [Member] | Q4 [Member] | NYMEX [Member]
Derivative [Line Items]
Volume per Day	58,000
Floor price	80.00
Ceiling price	97.73
Year 2014 [Member] | Q4 [Member] | Minimum [Member] | NYMEX [Member]
Derivative [Line Items]
Derivative Price Range	80.00
Year 2014 [Member] | Q4 [Member] | Maximum [Member] | NYMEX [Member]
Derivative [Line Items]
Derivative Price Range	100.00
Year 2015 [Member] | Q1 [Member] | NYMEX [Member]
Derivative [Line Items]
Volume per Day	38,000
Floor price	80.00
Ceiling price	96.96
Year 2015 [Member] | Q1 [Member] | LLS [Member]
Derivative [Line Items]
Volume per Day	20,000
Floor price	85.00
Ceiling price	101.45
Year 2015 [Member] | Q1 [Member] | Minimum [Member] | NYMEX [Member]
Derivative [Line Items]
Derivative Price Range	80.00
Year 2015 [Member] | Q1 [Member] | Minimum [Member] | LLS [Member]
Derivative [Line Items]
Derivative Price Range	85.00
Year 2015 [Member] | Q1 [Member] | Maximum [Member] | NYMEX [Member]
Derivative [Line Items]
Derivative Price Range	100.90
Year 2015 [Member] | Q1 [Member] | Maximum [Member] | LLS [Member]
Derivative [Line Items]
Derivative Price Range	104.00
Year 2015 [Member] | Q2 [Member] | NYMEX [Member]
Derivative [Line Items]
Volume per Day	24,000
Floor price	80.00
Ceiling price	94.40
Year 2015 [Member] | Q2 [Member] | LLS [Member]
Derivative [Line Items]
Volume per Day	20,000
Floor price	85.00
Ceiling price	102.01
Year 2015 [Member] | Q2 [Member] | Minimum [Member] | NYMEX [Member]
Derivative [Line Items]
Derivative Price Range	80.00
Year 2015 [Member] | Q2 [Member] | Minimum [Member] | LLS [Member]
Derivative [Line Items]
Derivative Price Range	85.00
Year 2015 [Member] | Q2 [Member] | Maximum [Member] | NYMEX [Member]
Derivative [Line Items]
Derivative Price Range	95.25
Year 2015 [Member] | Q2 [Member] | Maximum [Member] | LLS [Member]
Derivative [Line Items]
Derivative Price Range	103.00