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		Dec. 31, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Gross Amount		$ 16,440	$ 50
Netting Adjustments	[1]	(6,556)	(50)
Net Amount Presented in Balance Sheets		9,884
Gross Amount		(21,508)	(48,132)
Netting Adjustments	[1]	6,556	50
Net Amount Presented in Balance Sheets		(14,952)	(48,082)
Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		15,971	50
Netting Adjustments	[1]	(6,380)	(50)
Net Amount Presented in Balance Sheets		9,591
Other Noncurrent Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		469
Netting Adjustments	[1]	(176)
Net Amount Presented in Balance Sheets		293
Current Liabilities [Member] | Accrued Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		(21,256)	(35,459)
Netting Adjustments	[1]	6,380	50
Net Amount Presented in Balance Sheets		(14,876)	(35,409)
Noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		(252)	(12,673)
Netting Adjustments	[1]	176
Net Amount Presented in Balance Sheets		$ (76)	$ (12,673)

[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.