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, 2019	Dec. 31, 2018
Derivatives, Fair Value [Line Items]
Gross Amount		$ 25,516	$ 6,870
Netting Adjustments	[1]	(2,338)	(845)
Net Amount Presented in Balance Sheets		23,178	6,025
Gross Amount		(3,844)	(1,171)
Netting Adjustments	[1]	2,338	845
Net Amount Presented in Balance Sheets		(1,506)	(326)
Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		23,581	4,960
Netting Adjustments	[1]	(2,338)	(845)
Net Amount Presented in Balance Sheets		21,243	4,115
Other Noncurrent Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		1,935	1,910
Net Amount Presented in Balance Sheets		1,935	1,910
Current Liabilities [Member] | Accrued Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		(3,487)	(845)
Netting Adjustments	[1]	2,338	845
Net Amount Presented in Balance Sheets		(1,149)
Noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		(357)	(326)
Net Amount Presented in Balance Sheets		$ (357)	$ (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.