Financial Instruments - Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Assets
Derivative assets	$ 137	$ 1,423
Liabilities
Derivative liabilities	(7,052)	(2,288)
Fair Value, Measurements, Recurring [Member]
Assets
Derivative assets	137	1,423
Liabilities
Derivative liabilities	(7,052)	(2,288)
Commodity Derivative Contracts [Member] | Fair Value, Measurements, Recurring [Member]
Assets
Derivative assets	95	1,337
Liabilities
Derivative liabilities	(6,767)	(2,010)
Interest Rate Derivative Contracts [Member] | Fair Value, Measurements, Recurring [Member]
Assets
Derivative assets	42	86
Liabilities
Derivative liabilities	(285)	(278)
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]
Assets
Derivative assets	137	1,423
Liabilities
Derivative liabilities	(7,052)	(2,288)
Significant Unobservable Inputs (Level 3) [Member] | Commodity Derivative Contracts [Member] | Fair Value, Measurements, Recurring [Member]
Assets
Derivative assets	95	1,337
Liabilities
Derivative liabilities	(6,767)	(2,010)
Significant Unobservable Inputs (Level 3) [Member] | Interest Rate Derivative Contracts [Member] | Fair Value, Measurements, Recurring [Member]
Assets
Derivative assets	42	86
Liabilities
Derivative liabilities	$ (285)	$ (278)