Risk Management and Derivative Instruments - Gross Fair Value of Derivative Instruments by Appropriate Balance Sheet (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Derivative Instruments and Hedges, Assets [Abstract]
Asset Derivatives, Net recorded fair value	$ 0	$ 5,879
Asset Derivatives, Net recorded fair value	873	6,364
Liability Derivatives, Net recorded fair value	10,824	253
Liability Derivatives, Net recorded fair value	847	305
Short-Term Derivative Instruments [Member]
Derivative Instruments and Hedges, Assets [Abstract]
Total assets	6,088	11,766
Asset Derivatives, Netting arrangements	(6,088)	(5,887)
Asset Derivatives, Net recorded fair value	0	5,879
Liability Derivatives, Gross fair value	16,912	6,140
Liability Derivatives, Netting arrangements	(6,088)	(5,887)
Liability Derivatives, Net recorded fair value	10,824	253
Short-Term Derivative Instruments [Member] \| Commodity derivatives [Member]
Derivative Instruments and Hedges, Assets [Abstract]
Total assets	6,088	11,518
Liability Derivatives, Gross fair value	15,007	5,887
Short-Term Derivative Instruments [Member] \| Interest Rate Swaps [Member]
Derivative Instruments and Hedges, Assets [Abstract]
Total assets	0	248
Liability Derivatives, Gross fair value	1,905	253
Long-Term Derivative Instruments [Member]
Derivative Instruments and Hedges, Assets [Abstract]
Total assets	9,361	7,337
Asset Derivatives, Netting arrangements	(8,488)	(973)
Asset Derivatives, Net recorded fair value	873	6,364
Liability Derivatives, Gross fair value	9,335	1,278
Liability Derivatives, Netting arrangements	(8,488)	(973)
Liability Derivatives, Net recorded fair value	847	305
Long-Term Derivative Instruments [Member] \| Commodity derivatives [Member]
Derivative Instruments and Hedges, Assets [Abstract]
Total assets	9,361	6,990
Liability Derivatives, Gross fair value	8,488	973
Long-Term Derivative Instruments [Member] \| Interest Rate Swaps [Member]
Derivative Instruments and Hedges, Assets [Abstract]
Total assets	0	347
Liability Derivatives, Gross fair value	$ 847	$ 305

Fair value of liability associated with financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.

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