Derivative Instruments - Fair Value of Derivative Instruments on a Gross basis and on a Net Basis as Presented in Consolidated Balance Sheets (Detail) - Commodity Contract [Member] - Not Designated as Hedging Instrument [Member] - USD ($) $ in Thousands		Sep. 30, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Gross Amount		$ 41,045	$ 24,090
Netting Adjustments	[1]	(15,951)	(5,056)
Net Amount Presented in the Balance Sheets		25,094	19,034
Gross Amount		(15,951)	(5,056)
Netting Adjustments	[1]	15,951	5,056
Net Amount Presented in the Balance Sheets		0	0
Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		(9,399)	(5,012)
Netting Adjustments	[1]	9,399	5,012
Net Amount Presented in the Balance Sheets		0	0
Noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		(6,552)	(44)
Netting Adjustments	[1]	6,552	44
Net Amount Presented in the Balance Sheets		0	0
Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		33,148	22,349
Netting Adjustments	[1]	(9,399)	(5,012)
Net Amount Presented in the Balance Sheets		23,749	17,337
Other Noncurrent Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		7,897	1,741
Netting Adjustments	[1]	(6,552)	(44)
Net Amount Presented in the Balance Sheets		$ 1,345	$ 1,697

[1] The Company has agreements in place that allow for the financial right to offset for derivative assets and derivative liabilities at settlement or in the event of a default under the agreements.