Risk Management and Derivative Instruments - Gross Fair Value of Derivative Instruments by Appropriate Balance Sheet (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Derivative Instruments and Hedges, Assets [Abstract]
Asset Derivatives, Net recorded fair value	$ 20,709	$ 18,813
Asset Derivatives, Net recorded fair value	17,715	2,469
Liability Derivatives, Net recorded fair value	209	139
Liability Derivatives, Net recorded fair value	489	0
Short-Term Derivative Instruments [Member]
Derivative Instruments and Hedges, Assets [Abstract]
Asset Derivatives, Gross fair value	23,318	21,217
Asset Derivatives, Netting arrangements	(2,609)	(2,404)
Asset Derivatives, Net recorded fair value	20,709	18,813
Liability Derivatives, Gross fair value	2,818	2,543
Liability Derivatives, Netting arrangements	(2,609)	(2,404)
Liability Derivatives, Net recorded fair value	209	139
Short-Term Derivative Instruments [Member] \| Commodity derivatives [Member]
Derivative Instruments and Hedges, Assets [Abstract]
Asset Derivatives, Gross fair value	23,043	21,217
Liability Derivatives, Gross fair value	2,584	2,543
Short-Term Derivative Instruments [Member] \| Interest Rate Swaps [Member]
Derivative Instruments and Hedges, Assets [Abstract]
Asset Derivatives, Gross fair value	275	0
Liability Derivatives, Gross fair value	234	0
Long-Term Derivative Instruments [Member]
Derivative Instruments and Hedges, Assets [Abstract]
Asset Derivatives, Gross fair value	18,747	4,298
Asset Derivatives, Netting arrangements	(1,032)	(1,829)
Asset Derivatives, Net recorded fair value	17,715	2,469
Liability Derivatives, Gross fair value	1,521	1,829
Liability Derivatives, Netting arrangements	(1,032)	(1,829)
Liability Derivatives, Net recorded fair value	489	0
Long-Term Derivative Instruments [Member] \| Commodity derivatives [Member]
Derivative Instruments and Hedges, Assets [Abstract]
Asset Derivatives, Gross fair value	18,715	4,298
Liability Derivatives, Gross fair value	1,000	1,829
Long-Term Derivative Instruments [Member] \| Interest Rate Swaps [Member]
Derivative Instruments and Hedges, Assets [Abstract]
Asset Derivatives, Gross fair value	32	0
Liability Derivatives, Gross fair value	$ 521	$ 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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