Risk Management and Derivative Instruments - Gross Fair Value of Derivative Instruments by Appropriate Balance Sheet (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Derivative Instruments and Hedges, Assets [Abstract]
Asset Derivatives, Net recorded fair value	$ 5,919	$ 18,813
Asset Derivatives, Net recorded fair value	7,485	2,469
Liability Derivatives, Net recorded fair value	151	139
Liability Derivatives, Net recorded fair value	275	0
Short-Term Derivative Instruments [Member]
Derivative Instruments and Hedges, Assets [Abstract]
Asset Derivatives, Gross fair value	12,330	21,217
Asset Derivatives, Netting arrangements	(6,411)	(2,404)
Asset Derivatives, Net recorded fair value	5,919	18,813
Liability Derivatives, Gross fair value	6,562	2,543
Liability Derivatives, Netting arrangements	(6,411)	(2,404)
Liability Derivatives, Net recorded fair value	151	139
Short-Term Derivative Instruments [Member] \| Commodity derivatives [Member]
Derivative Instruments and Hedges, Assets [Abstract]
Asset Derivatives, Gross fair value	12,317	21,217
Liability Derivatives, Gross fair value	6,415	2,543
Short-Term Derivative Instruments [Member] \| Interest Rate Swaps [Member]
Derivative Instruments and Hedges, Assets [Abstract]
Asset Derivatives, Gross fair value	13	0
Liability Derivatives, Gross fair value	147	0
Long-Term Derivative Instruments [Member]
Derivative Instruments and Hedges, Assets [Abstract]
Asset Derivatives, Gross fair value	8,728	4,298
Asset Derivatives, Netting arrangements	(1,243)	(1,829)
Asset Derivatives, Net recorded fair value	7,485	2,469
Liability Derivatives, Gross fair value	1,518	1,829
Liability Derivatives, Netting arrangements	(1,243)	(1,829)
Liability Derivatives, Net recorded fair value	275	0
Long-Term Derivative Instruments [Member] \| Commodity derivatives [Member]
Derivative Instruments and Hedges, Assets [Abstract]
Asset Derivatives, Gross fair value	8,728	4,298
Liability Derivatives, Gross fair value	1,073	1,829
Long-Term Derivative Instruments [Member] \| Interest Rate Swaps [Member]
Derivative Instruments and Hedges, Assets [Abstract]
Asset Derivatives, Gross fair value	0	0
Liability Derivatives, Gross fair value	$ 445	$ 0

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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