Derivative Instruments and Hedging Activities - Open Crude and Natural Gas Derivative Instruments (Detail)	Jun. 30, 2013 MBbls
2013 [Member] | Fixed-Price Swaps [Member] | Derivative Asset Designated As Hedging Instruments [Member] | NYMEX WTI [Member] | Open Crude Oil Derivative Positions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Open positions	444
Weighted Average Fixed Price	77.66
2013 [Member] | Fixed-Price Swaps [Member] | Derivatives Not Designated as Cash Flow Hedging Instruments [Member] | Open Natural Gas Derivative Positions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Open positions	44,160
Weighted Average Fixed Price	4.20
2013 [Member] | Fixed-Price Swaps [Member] | Derivatives Not Designated as Cash Flow Hedging Instruments [Member] | NYMEX WTI [Member] | Open Crude Oil Derivative Positions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Open positions	11,040
Weighted Average Fixed Price	90.85
2013 [Member] | Fixed-Price Swaps [Member] | Derivatives Not Designated as Cash Flow Hedging Instruments [Member] | Dated Brent [Member] | Open Crude Oil Derivative Positions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Open positions	11,956
Weighted Average Fixed Price	106.47
2013 [Member] | Fixed-Price Swaps [Member] | Cash Flow Hedging [Member] | Open Natural Gas Derivative Positions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Open positions	5,034
Weighted Average Fixed Price	6.71
2013 [Member] | Collars [Member] | Derivative Asset Designated As Hedging Instruments [Member] | NYMEX WTI [Member] | Open Crude Oil Derivative Positions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Open positions	1,058
Collars Weighted Average Floor Price	78.48
Weighted Average Ceiling Price	103.20
2013 [Member] | Collars [Member] | Derivative Asset Designated As Hedging Instruments [Member] | Dated Brent [Member] | Open Crude Oil Derivative Positions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Open positions	1,656
Collars Weighted Average Floor Price	86.39
Weighted Average Ceiling Price	117.93
2013 [Member] | Collars [Member] | Cash Flow Hedging [Member] | Open Natural Gas Derivative Positions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Open positions	2,300
Collars Weighted Average Floor Price	5.35
Weighted Average Ceiling Price	6.67
2014 [Member] | Fixed-Price Swaps [Member] | Derivative Asset Designated As Hedging Instruments [Member] | NYMEX WTI [Member] | Open Crude Oil Derivative Positions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Open positions	76
Weighted Average Fixed Price	74.50
2014 [Member] | Fixed-Price Swaps [Member] | Derivatives Not Designated as Cash Flow Hedging Instruments [Member] | NYMEX WTI [Member] | Open Crude Oil Derivative Positions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Open positions	22,813
Weighted Average Fixed Price	90.83
2014 [Member] | Fixed-Price Swaps [Member] | Derivatives Not Designated as Cash Flow Hedging Instruments [Member] | Dated Brent [Member] | Open Crude Oil Derivative Positions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Open positions	22,812
Weighted Average Fixed Price	100.05
2014 [Member] | Fixed-Price Swaps [Member] | Cash Flow Hedging [Member] | Open Natural Gas Derivative Positions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Open positions	1,295
Weighted Average Fixed Price	6.72