Supplemental Information on Oil and Natural Gas Operations - Unaudited - Financial Data Related to Oil and Gas Producing Activities (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Accumulated DD&A:
Balance, beginning of year			$ (5,908,760)
Provision for DD&A			(340,006)		(350,574)		(344,365)
Write-down of oil and gas properties	304,062	47,130	351,192
Balance, end of year	(6,970,631)		(6,970,631)		(5,908,760)
Net proved oil and gas properties	1,846,637		1,846,637		1,895,357
Lease operating expenses			176,495		201,153		215,003
Transportation, processing and gathering expenses			64,951		42,172		21,782
Production taxes			12,151		15,029		10,015
Accretion expense			28,411		33,575		33,331
United States [Member]
Oil and gas properties, proved and unevaluated:
Balance, beginning of year			8,517,873		7,692,261		7,049,777
Costs incurred during the year (capitalized):
Acquisition costs, net of sales of unevaluated properties			44,634		70,903		102,807
Exploratory costs			270,850		297,113		81,458
Development costs			438,334	[1]	378,242	[1]	395,555	[1]
Salaries, general and administrative costs			33,975		32,815		25,318
Interest			45,722		46,860		37,656
Less: overhead reimbursements			(3,334)		(321)		(310)
Total costs incurred during the year			830,181		825,612		642,484
Balance, end of year	9,348,054		9,348,054		8,517,873		7,692,261
Accumulated DD&A:
Balance, beginning of year			(5,908,760)		(5,510,166)		(5,174,729)
Provision for DD&A			(335,987)		(346,827)		(341,096)
Write-down of oil and gas properties			(351,192)
Sale of proved properties			(374,692)		(51,767)		5,659
Balance, end of year	(6,970,631)		(6,970,631)		(5,908,760)		(5,510,166)
Net proved oil and gas properties	2,377,423		2,377,423		2,609,113		2,182,095
DD&A per Mcfe	3.59		3.59		3.43		3.69
Lease operating expenses			176,495		201,153		215,003
Transportation, processing and gathering expenses			64,951		42,172		21,782
Production taxes			12,151		15,029		10,015
Accretion expense			28,411		33,575		33,331
Expensed costs - United States			282,008		291,929		280,131
Canada [Member]
Oil and gas properties, proved and unevaluated:
Balance, beginning of year			10,583
Costs incurred during the year (capitalized):
Acquisition costs			6,956		8,764
Exploratory costs			19,040		1,819
Total costs incurred during the year			25,996		10,583
Balance, end of year	$ 36,579		$ 36,579		$ 10,583

[1] Includes capitalized asset retirement costs of ($20,305), $54,737 and $95,293, respectively.