Supplemental Information about Oil & Gas Producing Activities (Unaudited) (Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Discounted Future Net Cash Flows Relating to Proved Oil and Natural Gas Reserves
Standardized measure used for discounted future net cash flows to determine average price (in months)	12 months
Future cash inflows	$ 19,568,628		$ 14,354,627		$ 9,028,991
Future production costs	(5,226,044)		(4,446,183)		(3,826,832)
Future development costs	(1,975,429)		(1,789,001)		(1,159,465)
Future income tax expense	(3,770,512)		(2,272,184)		(969,771)
Future net cash flows	8,596,643		5,847,259		3,072,923
10% annual discount for estimated timing of cash flows	(4,561,364)		(3,048,103)		(1,627,176)
Standardized measure of discounted future net cash flows	4,035,279		2,799,156		1,445,747
BOE (average sales price in dollars)	71.18	[1]	52.93	[1]	38.37	[1]
Increase (Decrease) in Standardized Measure of Discounted Future Net Cash Flow Relating to Proved Oil and Gas Reserves [Roll Forward]
Standardized measure—beginning of year	2,799,156		1,445,747		1,135,581
Sales of oil and natural gas produced, net of production costs	(599,679)		(406,390)		(353,052)
Net changes in sales prices and production costs	1,473,454		1,724,212		637,882
Revisions of previous quantity estimates	(281,765)		(49,784)		(33,943)
Improved recovery	0		24,033		0
Extensions and discoveries	601,313		283,011		206,542
Change in estimated future development costs	(274,122)		(152,096)		(52,824)
Purchases of reserves in place	164,383		307,205		29,348
Sales of reserves in place	0		0		(138,265)
Development costs incurred during the period	433,660		144,086		110,200
Accretion of discount	383,418		184,917		131,745
Income taxes	(634,747)		(593,272)		(190,727)
Other	(29,792)		(112,513)		(36,740)
Net increase (decrease)	1,236,123		1,353,409		310,166
Standardized measure—end of year	$ 4,035,279		$ 2,799,156		$ 1,445,747
Oil ($/Bbl)
Discounted Future Net Cash Flows Relating to Proved Oil and Natural Gas Reserves
Average sales prices (in dollars per production type)	93.72	[1]	69.04	[1]	52.06	[1]
Natural Gas ($/Mcf)
Discounted Future Net Cash Flows Relating to Proved Oil and Natural Gas Reserves
Average sales prices (in dollars per production type)	4.02	[1]	4.57	[1]	3.58	[1]

[1] Differences between the average benchmark prices and the average prices used in the calculation of the standardized measure are attributable to adjustments made for transportation, quality and basis differentials.