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, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Gross Amount		$ 7,334	$ 50
Netting Adjustments	[1]	(4,469)	(50)
Net Amount Presented in Balance Sheets		2,865
Gross Amount		(12,850)	(48,132)
Netting Adjustments	[1]	4,469	50
Net Amount Presented in Balance Sheets		(8,381)	(48,082)
Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		6,358	50
Netting Adjustments	[1]	(3,638)	(50)
Net Amount Presented in Balance Sheets		2,720
Other Noncurrent Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		976
Netting Adjustments	[1]	(831)
Net Amount Presented in Balance Sheets		145
Current Liabilities [Member] | Accrued Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		(9,844)	(35,459)
Netting Adjustments	[1]	3,638	50
Net Amount Presented in Balance Sheets		(6,206)	(35,409)
Noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		(3,006)	(12,673)
Netting Adjustments	[1]	831
Net Amount Presented in Balance Sheets		$ (2,175)	$ (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.