Financial Instruments - Effect of Derivative Instruments on Condensed Consolidated Statements of Operations (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain/loss recognized in income	$ (8,854)	$ 2,471
Derivatives Designated as Cash-Flow Hedging Instruments [Member] | Interest Rate Swap [Member] | Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain/loss recognized in income	(132)
Derivatives Not Designated as Cash-Flow Hedging Instruments [Member] | Natural Gas Commodity Contracts [Member] | Unrealized Gain (Loss) on Derivative Instruments, Net [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain/loss recognized in income	(316)	(437)
Derivatives Not Designated as Cash-Flow Hedging Instruments [Member] | Natural Gas Commodity Contracts [Member] | Realized Gain (Loss) on Derivative Instruments, Net [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain/loss recognized in income	(596)	331
Derivatives Not Designated as Cash-Flow Hedging Instruments [Member] | Crude Oil Commodity Contracts [Member] | Unrealized Gain (Loss) on Derivative Instruments, Net [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain/loss recognized in income	(5,683)	2,834
Derivatives Not Designated as Cash-Flow Hedging Instruments [Member] | Crude Oil Commodity Contracts [Member] | Realized Gain (Loss) on Derivative Instruments, Net [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain/loss recognized in income	$ (2,127)	$ (257)