Derivative Instruments and Hedging Activities - Open Crude and Natural Gas Derivative Instruments (Detail)	Sep. 30, 2013 MMBTus
Fixed-Price Swaps [Member] | 2013 [Member] | Derivative Asset Designated As Hedging Instruments [Member] | Nymex Henry Hub [Member] | Open Natural Gas Derivative Positions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Open positions	2,517
Weighted Average Fixed Price	6.71
Fixed-Price Swaps [Member] | 2013 [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	222
Weighted Average Fixed Price	77.66
Fixed-Price Swaps [Member] | 2013 [Member] | Derivatives Not Designated as Cash Flow Hedging Instruments [Member] | Nymex Henry Hub [Member] | Open Natural Gas Derivative Positions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Open positions	22,080
Weighted Average Fixed Price	4.20
Fixed-Price Swaps [Member] | 2013 [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	5,520
Weighted Average Fixed Price	90.85
Fixed-Price Swaps [Member] | 2013 [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	5,978
Weighted Average Fixed Price	106.47
Fixed-Price Swaps [Member] | 2014 [Member] | Derivative Asset Designated As Hedging Instruments [Member] | Nymex Henry Hub [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
Fixed-Price Swaps [Member] | 2014 [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
Fixed-Price Swaps [Member] | 2014 [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
Fixed-Price Swaps [Member] | 2014 [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
Collars [Member] | 2013 [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	368
Weighted Average Floor Price	80.00
Weighted Average Ceiling Price	103.64
Collars [Member] | 2013 [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	828
Weighted Average Floor Price	86.39
Weighted Average Ceiling Price	117.93