Supplemental Information on Natural Gas and Oil Exploration, Development and Production Activities (Unaudited) - Additional Information (Detail) $ in Millions		12 Months Ended
		Dec. 31, 2017 USD ($) MMcfe $ / MMBTU $ / bbl $ / Mcf MMcf MBbls	Dec. 31, 2016 MMcfe $ / MMBTU $ / bbl $ / Mcf MMcf MBbls	Dec. 31, 2015 MMcfe $ / MMBTU $ / bbl $ / Mcf MMcf MBbls	Dec. 31, 2014 MMcfe MMcf MBbls
Reserve Quantities [Line Items]
Percentage of proved resources reviewed by consultants		98.00%
Statement That Reserve Estimates Differ From Auditors		more than 10%
Proved Reserve And Pretax Present Value of Reserve Discounted		10.00%
Variance in reserve estimates		less than 5%.
Scheduled to be drilling period maximum for undeveloped reserves		Scheduled to be drilled within five years
Reserve attributable to natural gas		82.00%	86.00%	80.00%
Revisions of positive pricing		46,000
Improved recoveries		597,000	393,000	781,000
Proved undeveloped reserves dropped		668,000	269,000	1,200,000
Cost spent on undeveloped reserves transferred to developed reserves | $		$ 920
Estimated future development costs in 2018 | $		717
Estimated future development costs in 2019 | $		707
Estimated future development costs in 2020 | $		$ 567
Proved undeveloped reserves reported for more than five years		64,000
Maximum
Reserve Quantities [Line Items]
Percentage of proved undeveloped reserves reported for more than five years		1.00%
Crude Oil and NGL
Reserve Quantities [Line Items]
Benchmark price used for calculating average realized prices | $ / bbl		51.19	42.68	50.13
Crude Oil
Reserve Quantities [Line Items]
Estimate reserve information average realized prices | $ / bbl		45.73	37.41	35.07
Proved developed and undeveloped reserves | MBbls		69,854	70,252	53,193	48,658
NGLs
Reserve Quantities [Line Items]
Estimate reserve information average realized prices | $ / bbl		17.84	13.44	11.74
Proved developed and undeveloped reserves | MBbls		763,264	630,066	549,135	515,907
Natural Gas
Reserve Quantities [Line Items]
Estimate reserve information average realized prices | $ / Mcf		2.60	2.07	2.07
Proved developed and undeveloped reserves | MMcf		10,263,649	7,870,416	6,277,697	6,922,836
Natural Gas, Per Thousand Cubic Feet
Reserve Quantities [Line Items]
Benchmark price used for calculating average realized prices | $ / MMBTU		2.98	2.48	2.59
Natural Gas Equivalents
Reserve Quantities [Line Items]
Proved reserves from drilling activities and evaluations of proved areas	[1]	3,487,519	1,394,134	1,265,348
Proved developed and undeveloped reserve balances	[1]	15,262,361	12,072,322	9,891,663	10,310,229
Revisions of previous estimates	[1]	506,919	255,794	(211,163)
Revisions of positive performance		532,000	154,000
Ethane and Natural Gas Equivalents
Reserve Quantities [Line Items]
Proved developed and undeveloped reserve balances		1,596,000	1,367,000	1,296,000
Ethane Reserves and NGLs
Reserve Quantities [Line Items]
Proved developed and undeveloped reserves | MBbls		360,600	308,900	292,800

[1] Oil and NGLs are converted to mcfe at the rate of one barrel equals six mcf based upon the approximate relative energy content of oil to natural gas, which is not indicative of the relationship of oil and natural gas prices.