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, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Gross Amount		$ 8,192	$ 16,440
Netting Adjustments	[1]	(3,096)	(6,556)
Net Amount Presented in Balance Sheets		5,096	9,884
Gross Amount		(29,277)	(21,508)
Netting Adjustments	[1]	3,096	6,556
Net Amount Presented in Balance Sheets		(26,181)	(14,952)
Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		6,573	15,971
Netting Adjustments	[1]	(3,089)	(6,380)
Net Amount Presented in Balance Sheets		3,484	9,591
Other Noncurrent Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		1,619	469
Netting Adjustments	[1]	(7)	(176)
Net Amount Presented in Balance Sheets		1,612	293
Current Liabilities [Member] | Accrued Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		(26,345)	(21,256)
Netting Adjustments	[1]	3,089	6,380
Net Amount Presented in Balance Sheets		(23,256)	(14,876)
Noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		(2,932)	(252)
Netting Adjustments	[1]	7	176
Net Amount Presented in Balance Sheets		$ (2,925)	$ (76)

[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.