Derivative Instrument and Hedging Activities - Open Crude and Natural Gas Derivative Instruments (Detail)	Mar. 31, 2013 MBbls
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	1,212
Weighted Average Fixed Price	74.90
Fixed-Price Swaps [Member] | 2013 [Member] | Derivatives Not Designated As Cash Flow Hedging Instruments [Member] | Open Crude Oil Derivative Positions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Open positions	34,370
Weighted Average Fixed Price	98.97
Fixed-Price Swaps [Member] | 2013 [Member] | Cash Flow Hedging [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	7,575,000
Weighted Average Fixed Price	6.73
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
Weighted Average Fixed Price	74.50
Fixed-Price Swaps [Member] | 2014 [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	1,295,000
Weighted Average Fixed Price	6.72
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	4,216
Collars Weighted Average Floor Price	82.99
Weighted Average Ceiling Price	111.81
Collars [Member] | 2013 [Member] | Cash Flow Hedging [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	4,575,000
Collars Weighted Average Floor Price	5.35
Weighted Average Ceiling Price	6.67