Risk Management and Derivative Instruments - Gross Fair Value of Derivative Instruments by Appropriate Balance Sheet (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Derivative Instruments and Hedges, Assets [Abstract]
Asset Derivatives, Net recorded fair value	$ 69,464	$ 272,320
Asset Derivatives, Net recorded fair value	102,630	461,810
Liability Derivatives, Net recorded fair value	0	2,850
Liability Derivatives, Net recorded fair value	0	1,441
Short-Term Derivative Instruments [Member]
Derivative Instruments and Hedges, Assets [Abstract]
Asset Derivatives, Gross fair value	86,335	324,265
Asset Derivatives, Netting arrangements	(16,871)	(51,945)
Asset Derivatives, Net recorded fair value	69,464	272,320
Liability Derivatives, Gross fair value	16,871	54,795
Liability Derivatives, Netting arrangements	(16,871)	(51,945)
Liability Derivatives, Net recorded fair value	0	2,850
Short-Term Derivative Instruments [Member] | Commodity derivatives [Member]
Derivative Instruments and Hedges, Assets [Abstract]
Asset Derivatives, Gross fair value	86,335	324,265
Liability Derivatives, Gross fair value	16,871	53,581
Short-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	0	1,214
Long-Term Derivative Instruments [Member]
Derivative Instruments and Hedges, Assets [Abstract]
Asset Derivatives, Gross fair value	103,515	492,730
Asset Derivatives, Netting arrangements	(885)	(30,920)
Asset Derivatives, Net recorded fair value	102,630	461,810
Liability Derivatives, Gross fair value	885	32,361
Liability Derivatives, Netting arrangements	(885)	(30,920)
Liability Derivatives, Net recorded fair value	0	1,441
Long-Term Derivative Instruments [Member] | Commodity derivatives [Member]
Derivative Instruments and Hedges, Assets [Abstract]
Asset Derivatives, Gross fair value	103,515	492,730
Liability Derivatives, Gross fair value	885	30,920
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	$ 0	$ 1,441