Derivative Instruments and Hedging Activities - Commodity Derivative Instruments (Detail) (Fixed-Price Swaps [Member], 2014 [Member], Derivatives Not Designated as Cash Flow Hedging Instruments [Member])	Mar. 31, 2014 MMBTU
Open Natural Gas Derivative Positions [Member] | NYMEX Henry Hub and Various Inside FERC Indices [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Open positions	48,835,000
Weighted Average Fixed Price	4.39
Open Crude Oil Derivative Positions [Member] | NYMEX WTI [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Open positions	17,218
Weighted Average Fixed Price	90.80
Open Crude Oil Derivative Positions [Member] | Dated Brent [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Open positions	17,188
Weighted Average Fixed Price	100.05