Derivative Instruments - Fair Value of Derivative Instruments on a Gross basis and on a Net Basis as Presented in Consolidated Balance Sheets (Detail) - Not Designated as Hedging Instrument [Member] - USD ($) $ in Thousands		Jun. 30, 2020	Dec. 31, 2019
Derivatives, Fair Value [Line Items]
Gross Amount		$ (24,548)
Netting Adjustments	[1]	6,510
Net Amount Presented in Balance Sheets		(18,038)
Commodity Contract [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		36,444	$ 34,595
Netting Adjustments	[1]	(6,510)	(3,764)
Net Amount Presented in Balance Sheets		29,934	30,831
Gross Amount			(7,478)
Netting Adjustments	[1]		3,764
Net Amount Presented in Balance Sheets			(3,714)
Commodity Contract [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		36,444	33,762
Netting Adjustments	[1]	(6,510)	(3,719)
Net Amount Presented in Balance Sheets		29,934	30,043
Commodity Contract [Member] | Current Liabilities [Member] | Accrued Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		(7,514)	(5,081)
Netting Adjustments	[1]	6,510	3,719
Net Amount Presented in Balance Sheets		(1,004)	(1,362)
Commodity Contract [Member] | Noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		(16,437)	(2,397)
Netting Adjustments	[1]		45
Net Amount Presented in Balance Sheets		(16,437)	(2,352)
Commodity Contract [Member] | Other Noncurrent Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amount			833
Netting Adjustments	[1]		(45)
Net Amount Presented in Balance Sheets			$ 788
Interest-Rate Swap [Member] | Current Liabilities [Member] | Accrued Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		(101)
Net Amount Presented in Balance Sheets		(101)
Interest-Rate Swap [Member] | Noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		(496)
Net Amount Presented in Balance Sheets		$ (496)

[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.