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, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Gross Amount		$ 6,870	$ 16,440
Netting Adjustments	[1]	(845)	(6,556)
Net Amount Presented in Balance Sheets		6,025	9,884
Gross Amount		(1,171)	(21,508)
Netting Adjustments	[1]	845	6,556
Net Amount Presented in Balance Sheets		(326)	(14,952)
Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		4,960	15,971
Netting Adjustments	[1]	(845)	(6,380)
Net Amount Presented in Balance Sheets		4,115	9,591
Other Noncurrent Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		1,910	469
Netting Adjustments	[1]		(176)
Net Amount Presented in Balance Sheets		1,910	293
Current Liabilities [Member] | Accrued Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		(845)	(21,256)
Netting Adjustments	[1]	845	6,380
Net Amount Presented in Balance Sheets			(14,876)
Noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		(326)	(252)
Netting Adjustments	[1]		176
Net Amount Presented in Balance Sheets		$ (326)	$ (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.