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		Mar. 31, 2019	Dec. 31, 2018
Derivatives, Fair Value [Line Items]
Gross Amount		$ 3,926	$ 6,870
Netting Adjustments	[1]	(2,330)	(845)
Net Amount Presented in Balance Sheets		1,596	6,025
Gross Amount		(7,206)	(1,171)
Netting Adjustments	[1]	2,330	845
Net Amount Presented in Balance Sheets		(4,876)	(326)
Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		2,416	4,960
Netting Adjustments	[1]	(2,154)	(845)
Net Amount Presented in Balance Sheets		262	4,115
Other Noncurrent Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		1,510	1,910
Netting Adjustments	[1]	(176)
Net Amount Presented in Balance Sheets		1,334	1,910
Current Liabilities [Member] | Accrued Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		(6,234)	(845)
Netting Adjustments	[1]	2,154	845
Net Amount Presented in Balance Sheets		(4,080)
Noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		(972)	(326)
Netting Adjustments	[1]	176
Net Amount Presented in Balance Sheets		$ (796)	$ (326)

[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.