Risk Management and Derivative Instruments - Gross Fair Value of Derivative Instruments by Appropriate Balance Sheet (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Derivative Instruments and Hedges, Assets [Abstract]
Asset Derivatives, Net recorded fair value	$ 9,916	$ 5,879
Asset Derivatives, Net recorded fair value	3,014	6,364
Liability Derivatives, Net recorded fair value	786	253
Liability Derivatives, Net recorded fair value	1,350	305
Short-Term Derivative Instruments [Member]
Derivative Instruments and Hedges, Assets [Abstract]
Asset Derivatives, Gross fair value	20,084	11,766
Asset Derivatives, Netting arrangements	(10,168)	(5,887)
Asset Derivatives, Net recorded fair value	9,916	5,879
Liability Derivatives, Gross fair value	10,954	6,140
Liability Derivatives, Netting arrangements	(10,168)	(5,887)
Liability Derivatives, Net recorded fair value	786	253
Short-Term Derivative Instruments [Member] \| Commodity derivatives [Member]
Derivative Instruments and Hedges, Assets [Abstract]
Asset Derivatives, Gross fair value	20,084	11,518
Liability Derivatives, Gross fair value	9,092	5,887
Short-Term Derivative Instruments [Member] \| Interest Rate Swaps [Member]
Derivative Instruments and Hedges, Assets [Abstract]
Asset Derivatives, Gross fair value	0	248
Liability Derivatives, Gross fair value	1,862	253
Long-Term Derivative Instruments [Member]
Derivative Instruments and Hedges, Assets [Abstract]
Asset Derivatives, Gross fair value	7,178	7,337
Asset Derivatives, Netting arrangements	(4,164)	(973)
Asset Derivatives, Net recorded fair value	3,014	6,364
Liability Derivatives, Gross fair value	5,514	1,278
Liability Derivatives, Netting arrangements	(4,164)	(973)
Liability Derivatives, Net recorded fair value	1,350	305
Long-Term Derivative Instruments [Member] \| Commodity derivatives [Member]
Derivative Instruments and Hedges, Assets [Abstract]
Asset Derivatives, Gross fair value	7,178	6,990
Liability Derivatives, Gross fair value	4,164	973
Long-Term Derivative Instruments [Member] \| Interest Rate Swaps [Member]
Derivative Instruments and Hedges, Assets [Abstract]
Asset Derivatives, Gross fair value	0	347
Liability Derivatives, Gross fair value	$ 1,350	$ 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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Fair value, after the effects of master netting arrangements, of a 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, expected to be settled within one year or normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset.

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