Financial Data Related to Oil and Gas Producing Activities (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Accumulated depreciation, depletion and amortization (DD&A):
Balance, beginning of year	$ (5,174,729)
Provision for DD&A	(344,365)		(280,020)		(248,201)
Balance, end of year	(5,510,166)		(5,174,729)
Net capitalized costs, proved and unevaluated	1,734,300		1,473,439
Lease operating expenses	215,003		175,881		150,212
Transportation, processing and gathering expenses	21,782		8,958		7,218
Production taxes	10,015		9,380		5,808
Accretion expense	33,331		30,764		34,469
United States
Oil and gas properties, proved and unevaluated:
Balance, beginning of year	7,049,777		6,202,758		5,741,081
Costs incurred during the year (capitalized):
Acquisition costs, net of sales of unevaluated properties	102,807		270,354		127,069
Exploratory costs	81,458		84,199		42,205
Development costs	395,555	[1]	426,355	[1]	241,387	[1]
Salaries, general and administrative costs	25,318		24,430		20,521
Interest	37,656		42,033		30,783
Less: overhead reimbursements	(310)		(352)		(288)
Total costs incurred during the year, net of divestitures	642,484		847,019		461,677
Balance, end of year	7,692,261		7,049,777		6,202,758
Accumulated depreciation, depletion and amortization (DD&A):
Balance, beginning of year	(5,174,729)		(4,804,949)		(4,555,372)
Provision for DD&A	(341,096)		(276,480)		(242,745)
Sale of proved properties	5,659		(93,300)		(6,832)
Balance, end of year	(5,510,166)		(5,174,729)		(4,804,949)
Net capitalized costs, proved and unevaluated	2,182,095		1,875,048		1,397,809
DD&A per Mcfe	3.69		3.45		3.08
Lease operating expenses	215,003		175,881		150,212
Transportation, processing and gathering expenses	21,782		8,958		7,218
Production taxes	10,015		9,380		5,808
Accretion expense	33,331		30,764		34,469
Expensed costs	$ 280,131		$ 224,983		$ 197,707

[1] Includes capitalized asset retirement costs of $95,293, $96,386 and $56,444, respectively.