Derivative Instruments and Hedging Activities (Oustanding commodity derivative contracts) (Details 1)	Dec. 31, 2012
Year 2013 [Member] | Q1 [Member]
Derivative [Line Items]
Weighted average floor price	78.91
Weighted average ceiling price	108.01
Volume per day (Bbl per day)	55,000
Year 2013 [Member] | Q2 [Member]
Derivative [Line Items]
Weighted average floor price	79.64
Weighted average ceiling price	108.61
Volume per day (Bbl per day)	56,000
Year 2013 [Member] | Q3 [Member]
Derivative [Line Items]
Weighted average floor price	79.64
Weighted average ceiling price	109.15
Volume per day (Bbl per day)	56,000
Year 2013 [Member] | Q4 [Member]
Derivative [Line Items]
Weighted average floor price	80.00
Weighted average ceiling price	117.53
Volume per day (Bbl per day)	54,000
Year 2014 [Member] | Q1 [Member]
Derivative [Line Items]
Weighted average floor price	80.00
Weighted average ceiling price	102.44
Volume per day (Bbl per day)	52,000
Year 2014 [Member] | Q2 [Member]
Derivative [Line Items]
Weighted average floor price	80.00
Weighted average ceiling price	102.44
Volume per day (Bbl per day)	52,000
Year 2014 [Member] | Q3 [Member]
Derivative [Line Items]
Weighted average floor price	80.00
Weighted average ceiling price	97.46
Volume per day (Bbl per day)	48,000
Year 2014 [Member] | Q4 [Member]
Derivative [Line Items]
Weighted average floor price	80.00
Weighted average ceiling price	97.46
Volume per day (Bbl per day)	48,000
Minimum [Member] | Year 2013 [Member] | Q1 [Member]
Derivative [Line Items]
Derivative Price Range	70.00
Minimum [Member] | Year 2013 [Member] | Q2 [Member]
Derivative [Line Items]
Derivative Price Range	75.00
Minimum [Member] | Year 2013 [Member] | Q3 [Member]
Derivative [Line Items]
Derivative Price Range	75.00
Minimum [Member] | Year 2013 [Member] | Q4 [Member]
Derivative [Line Items]
Derivative Price Range	80.00
Minimum [Member] | Year 2014 [Member] | Q1 [Member]
Derivative [Line Items]
Derivative Price Range	80.00
Minimum [Member] | Year 2014 [Member] | Q2 [Member]
Derivative [Line Items]
Derivative Price Range	80.00
Minimum [Member] | Year 2014 [Member] | Q3 [Member]
Derivative [Line Items]
Derivative Price Range	80.00
Minimum [Member] | Year 2014 [Member] | Q4 [Member]
Derivative [Line Items]
Derivative Price Range	80.00
Maximum [Member] | Year 2013 [Member] | Q1 [Member]
Derivative [Line Items]
Derivative Price Range	113.00
Maximum [Member] | Year 2013 [Member] | Q2 [Member]
Derivative [Line Items]
Derivative Price Range	121.50
Maximum [Member] | Year 2013 [Member] | Q3 [Member]
Derivative [Line Items]
Derivative Price Range	133.10
Maximum [Member] | Year 2013 [Member] | Q4 [Member]
Derivative [Line Items]
Derivative Price Range	127.50
Maximum [Member] | Year 2014 [Member] | Q1 [Member]
Derivative [Line Items]
Derivative Price Range	104.50
Maximum [Member] | Year 2014 [Member] | Q2 [Member]
Derivative [Line Items]
Derivative Price Range	104.50
Maximum [Member] | Year 2014 [Member] | Q3 [Member]
Derivative [Line Items]
Derivative Price Range	98.80
Maximum [Member] | Year 2014 [Member] | Q4 [Member]
Derivative [Line Items]
Derivative Price Range	98.80