Derivative Instrument and Hedging Activities - Open Crude and Natural Gas Derivative Instruments (Detail)	Dec. 31, 2012 MMBTU
Fixed-Price Swaps [Member] | 2013 [Member] | Open Natural Gas Derivative Positions [Member] | Currency Swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Open positions	10,095,000
Weighted Average Fixed Price	6.74
Fixed-Price Swaps [Member] | 2013 [Member] | Open Crude Oil Derivative Positions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Open positions	40,292,000
Weighted Average Fixed Price	97.31
Fixed-Price Swaps [Member] | 2014 [Member] | Open Natural Gas Derivative Positions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Open positions	1,295,000
Weighted Average Fixed Price	6.72
Fixed-Price Swaps [Member] | 2014 [Member] | Open Crude Oil Derivative Positions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Open positions	76,000
Weighted Average Fixed Price	74.50
Collars [Member] | 2013 [Member] | Open Natural Gas Derivative Positions [Member] | Currency Swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Open positions	6,825,000
Collars Weighted Average Floor Price	5.35
Weighted Average Ceiling Price	6.67
Collars [Member] | 2013 [Member] | Open Crude Oil Derivative Positions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Open positions	5,701
Collars Weighted Average Floor Price	82.84
Weighted Average Ceiling Price	111.63
Collars [Member] | 2014 [Member] | Open Crude Oil Derivative Positions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Open positions
Collars Weighted Average Floor Price
Weighted Average Ceiling Price