Risk Management and Derivative Instruments - Gross Fair Value of Derivative Instruments by Appropriate Balance Sheet (Detail) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Derivative Instruments and Hedges, Assets [Abstract]
Asset Derivatives, Net recorded fair value	$ 0	$ 873
Liability Derivatives, Net recorded fair value	63,071	10,824
Liability Derivatives, Net recorded fair value	17,739	847
Short-Term Derivative Instruments [Member]
Derivative Instruments and Hedges, Assets [Abstract]
Total assets	1,172	6,088
Asset Derivatives, Netting arrangements	(1,172)	(6,088)
Asset Derivatives, Net recorded fair value	0	0
Liability Derivatives, Gross fair value	64,243	16,912
Liability Derivatives, Netting arrangements	(1,172)	(6,088)
Liability Derivatives, Net recorded fair value	63,071	10,824
Short-Term Derivative Instruments [Member] | Commodity derivatives [Member]
Derivative Instruments and Hedges, Assets [Abstract]
Total assets	1,172	6,088
Liability Derivatives, Gross fair value	62,901	15,007
Short-Term Derivative Instruments [Member] | Interest Rate Swaps [Member]
Derivative Instruments and Hedges, Assets [Abstract]
Total assets	0	0
Liability Derivatives, Gross fair value	1,342	1,905
Long-Term Derivative Instruments [Member]
Derivative Instruments and Hedges, Assets [Abstract]
Total assets	4,124	9,361
Asset Derivatives, Netting arrangements	(4,124)	(8,488)
Asset Derivatives, Net recorded fair value	0	873
Liability Derivatives, Gross fair value	21,863	9,335
Liability Derivatives, Netting arrangements	(4,124)	(8,488)
Liability Derivatives, Net recorded fair value	17,739	847
Long-Term Derivative Instruments [Member] | Commodity derivatives [Member]
Derivative Instruments and Hedges, Assets [Abstract]
Total assets	4,124	9,361
Liability Derivatives, Gross fair value	21,436	8,488
Long-Term Derivative Instruments [Member] | Interest Rate Swaps [Member]
Derivative Instruments and Hedges, Assets [Abstract]
Total assets	0	0
Liability Derivatives, Gross fair value	$ 427	$ 847