Supplemental Information on Natural Gas and Oil Exploration, Development and Production Activities (Unaudited) - Additional Information (Detail) $ in Millions		12 Months Ended
		Dec. 31, 2015 USD ($) MMcfe $ / MMBTU $ / bbl $ / Mcf MMcf MBbls	Dec. 31, 2014 MMcfe $ / MMBTU $ / bbl $ / Mcf MMcf MBbls	Dec. 31, 2013 MMcfe $ / MMBTU $ / bbl $ / Mcf MMcf MBbls	Dec. 31, 2012 MMcfe MMcf MBbls
Reserve Quantities [Line Items]
Percentage of proved resources reviewed by consultant		94.00%
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		80.00%	58.00%	49.00%
Improved recoveries		781,000	449,600	630,300
Proved undeveloped reserves dropped		1,200,000	611,000
Cost spent on undeveloped reserves transferred to developed reserves | $		$ 398.8
Estimated future development costs in 2016 | $		435.6
Estimated future development costs in 2017 | $		509,400.0
Estimated future development costs in 2018 | $		$ 471.6
Crude Oil and NGL
Reserve Quantities [Line Items]
Benchmark price used for calculating average realized prices | $ / bbl		50.13	94.42	97.33
Crude Oil
Reserve Quantities [Line Items]
Estimate reserve information average realized prices | $ / bbl		35.07	79.04	86.66
Proved developed and undeveloped reserves | MBbls		53,193	48,658	48,360	45,082
NGLs
Reserve Quantities [Line Items]
Estimate reserve information average realized prices | $ / bbl		11.74	27.20	25.93
Proved developed and undeveloped reserves | MBbls		549,135	515,907	374,412	240,399
Natural Gas
Reserve Quantities [Line Items]
Estimate reserve information average realized prices | $ / Mcf		2.07	4.14	3.75
Proved developed and undeveloped reserves | MMcf		6,277,697	6,922,836	5,665,645	4,792,676
Natural Gas, Per Thousand Cubic Feet
Reserve Quantities [Line Items]
Benchmark price used for calculating average realized prices | $ / MMBTU		2.59	4.35	3.67
Natural Gas Equivalents
Reserve Quantities [Line Items]
Proved reserves from drilling activities and evaluations of proved areas	[1]	1,265,348	2,398,709	1,732,944
Proved developed and undeveloped reserve balances	[1]	9,891,663	10,310,229	8,202,274	6,505,570
Revisions of previous estimates	[1]	(211,163)	90,822	448,898
Ethane and Natural Gas Equivalents
Reserve Quantities [Line Items]
Proved developed and undeveloped reserve balances		1,296,000	1,170,000	676,000
Ethane Reserves and NGLs
Reserve Quantities [Line Items]
Proved developed and undeveloped reserves | MBbls		292,800	264,300	155,800

[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.