Financial Data Related to Oil and Gas Producing Activities (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Accumulated DD&A:
Balance, beginning of year	$ (5,510,166)
Provision for DD&A	(350,574)		(344,365)		(280,020)
Balance, end of year	(5,908,760)		(5,510,166)
Net proved oil and gas properties	1,895,357		1,734,300
Lease operating expenses	201,153		215,003		175,881
Transportation, processing and gathering expenses	42,172		21,782		8,958
Production taxes	15,029		10,015		9,380
Accretion expense	33,575		33,331		30,764
United States [Member]
Oil and gas properties, proved and unevaluated:
Balance, beginning of year	7,692,261		7,049,777		6,202,758
Costs incurred during the year (capitalized):
Acquisition costs, net of sales of unevaluated properties	70,903		102,807		270,354
Exploratory costs	297,113		81,458		84,199
Development costs	378,242	[1]	395,555	[1]	426,355	[1]
Salaries, general and administrative costs	32,815		25,318		24,430
Interest	46,860		37,656		42,033
Less: overhead reimbursements	(321)		(310)		(352)
Total costs incurred during the year	825,612		642,484		847,019
Balance, end of year	8,517,873		7,692,261		7,049,777
Accumulated DD&A:
Balance, beginning of year	(5,510,166)		(5,174,729)		(4,804,949)
Provision for DD&A	(346,827)		(341,096)		(276,480)
Sale of proved properties	(51,767)		5,659		(93,300)
Balance, end of year	(5,908,760)		(5,510,166)		(5,174,729)
Net proved oil and gas properties	2,609,113		2,182,095		1,875,048
DD&A per Mcfe	3.43		3.69		3.45
Lease operating expenses	201,153		215,003		175,881
Transportation, processing and gathering expenses	42,172		21,782		8,958
Production taxes	15,029		10,015		9,380
Accretion expense	33,575		33,331		30,764
Expensed costs - United States	291,929		280,131		224,983
Canada [Member]
Oil and gas properties, proved and unevaluated:
Balance, beginning of year
Costs incurred during the year (capitalized):
Acquisition costs	8,764
Exploratory costs	1,819
Total costs incurred during the year	10,583
Balance, end of year	$ 10,583

[1] Includes capitalized asset retirement costs of $54,737, $95,293 and $96,386, respectively.