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, 2019	Dec. 31, 2018
Derivatives, Fair Value [Line Items]
Gross Amount		$ 35,160	$ 6,870
Netting Adjustments	[1]	(3,978)	(845)
Net Amount Presented in Balance Sheets		31,182	6,025
Gross Amount		(7,036)	(1,171)
Netting Adjustments	[1]	3,978	845
Net Amount Presented in Balance Sheets		(3,058)	(326)
Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		32,344	4,960
Netting Adjustments	[1]	(3,279)	(845)
Net Amount Presented in Balance Sheets		29,065	4,115
Other Noncurrent Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		2,816	1,910
Netting Adjustments	[1]	(699)
Net Amount Presented in Balance Sheets		2,117	1,910
Current Liabilities [Member] | Accrued Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		(4,232)	(845)
Netting Adjustments	[1]	3,279	845
Net Amount Presented in Balance Sheets		(953)
Noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		(2,804)	(326)
Netting Adjustments	[1]	699
Net Amount Presented in Balance Sheets		$ (2,105)	$ (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.