Investment in Oil and Gas Properties (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2009	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Accumulated depreciation, depletion and amortization (DD&A) :
Balance, beginning of year		$ 4,804,949
Provision for DD&A		280,020	248,201	259,639
Write-down of oil and gas properties				508,989
Sale of proved properties		93,300
Balance, end of year		5,174,729	4,804,949
Net capitalized costs proved and unevaluated		1,473,439	984,629
United States [Member]
Oil and gas properties, proved and unevaluated:
Balance, beginning of year	5,409,770	6,202,758	5,741,081	5,409,770
Costs incurred during the year (capitalized):
Acquisition costs, net of sales of unevaluated properties		270,354	127,069	9,072
Exploratory costs		84,199	42,205	78,582
Development costs		426,355	241,387	199,375
Salaries, general and administrative costs		24,430	20,521	19,107
Interest		42,033	30,783	25,573
Less : overhead reimbursements		(352)	(288)	(398)
Total costs incurred during the year, net of divestitures		847,019	461,677	331,311
Balance, end of year		7,049,777	6,202,758	5,741,081
Accumulated depreciation, depletion and amortization (DD&A) :
Balance, beginning of year	3,781,600	4,804,949	4,555,372	3,781,600
Provision for DD&A		276,480	(242,745)	(253,790)
Write-down of oil and gas properties	343,932			508,989
Sale of proved properties			(6,832)	(10,993)
Balance, end of year		5,174,729	4,804,949	4,555,372
Net capitalized costs proved and unevaluated	$ 1,185,709	$ 1,875,048	$ 1,397,809
DD&A per Mcfe		3.54	3.18	3.23