Derivative Commodity Instruments - Hedging Transactions (Details) (Cash Flow Hedging)	6 Months Ended
	Jun. 30, 2013 Mcf
Natural Gas | 2013 | NYMEX Swaps
Derivative [Line Items]
Total Hedged Volumes	6,200,000
Average Contract Price, per Mcf	4.83
Natural Gas | 2013 | Basin Specific Swaps - San Juan
Derivative [Line Items]
Total Hedged Volumes	17,900,000
Average Contract Price, per Mcf	4.51
Natural Gas | 2013 | Basin Specific Swaps - Permian
Derivative [Line Items]
Total Hedged Volumes	3,100,000
Average Contract Price, per Mcf	3.64
Natural Gas | 2014 | NYMEX Swaps
Derivative [Line Items]
Total Hedged Volumes	10,600,000
Average Contract Price, per Mcf	4.55
Natural Gas | 2014 | Basin Specific Swaps - San Juan
Derivative [Line Items]
Total Hedged Volumes	31,400,000
Average Contract Price, per Mcf	4.60
Natural Gas | 2014 | Basin Specific Swaps - Permian
Derivative [Line Items]
Total Hedged Volumes	9,700,000
Average Contract Price, per Mcf	3.81
Natural Gas | 2015 | Basin Specific Swaps - San Juan
Derivative [Line Items]
Total Hedged Volumes	6,000,000
Average Contract Price, per Mcf	4.07
Crude Oil | 2013 | NYMEX Swaps
Derivative [Line Items]
Total Hedged Volumes	4,603,000
Average Contract Price, per Bbl	91.07
Crude Oil | 2013 | WTS/WTI Basis Swaps
Derivative [Line Items]
Total Hedged Volumes	1,811,000	[1]
Average Contract Price, per Bbl	(3.00)	[1]
Crude Oil | 2013 | WTI/WTI Basis Swaps
Derivative [Line Items]
Total Hedged Volumes	2,053,000	[2]
Average Contract Price, per Bbl	(1.00)	[2]
Crude Oil | 2014 | NYMEX Swaps
Derivative [Line Items]
Total Hedged Volumes	9,796,000
Average Contract Price, per Bbl	92.64
Crude Oil | 2015 | NYMEX Swaps
Derivative [Line Items]
Total Hedged Volumes	720,000
Average Contract Price, per Bbl	90.10
Natural Gas Liquids | 2013 | Liquids Swaps
Derivative [Line Items]
Total Hedged Volumes	23,400,000
Average Contract Price, per Bbl	1.02

[1] WTS - West Texas Sour/Midland, WTI - West Texas Intermediate/Cushing

[2] WTI - West Texas Intermediate/Midland, WTI - West Texas Intermediate/Cushing