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		Jun. 30, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Gross Amount		$ 4,822	$ 45,793
Netting Adjustments	[1]	(4,822)	(11,352)
Net Amount Presented in Balance Sheets			34,441
Gross Amount		(17,455)	(11,352)
Netting Adjustments	[1]	4,822	11,352
Net Amount Presented in Balance Sheets		(12,633)
Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		4,501	41,199
Netting Adjustments	[1]	(4,501)	(8,158)
Net Amount Presented in Balance Sheets			33,041
Other Noncurrent Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		321	4,594
Netting Adjustments	[1]	(321)	(3,194)
Net Amount Presented in Balance Sheets			1,400
Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amount			(8,158)
Netting Adjustments	[1]		8,158
Current Liabilities [Member] | Accrued Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		(7,703)
Netting Adjustments	[1]	4,501
Net Amount Presented in Balance Sheets		(3,202)
Noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		(9,752)	(3,194)
Netting Adjustments	[1]	321	$ 3,194
Net Amount Presented in Balance Sheets		$ (9,431)

[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.