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, 2012		Dec. 31, 2011		Dec. 31, 2010
Discounted Future Net Cash Flows Relating to Proved Oil and Natural Gas Reserves
Discount Rate Used in Standardized Measure of Discounted Future Net Cash Flow of Proved Oil and Gas Reserves	10.00%
Future cash inflows	$ 19,738,729		$ 19,568,628		$ 14,354,627
Future production costs	(5,884,891)		(5,226,044)		(4,446,183)
Future development costs	(2,164,780)		(1,975,429)		(1,789,001)
Future income tax expense	(3,344,024)		(3,770,512)		(2,272,184)
Future net cash flows	8,345,034		8,596,643		5,847,259
10% annual discount for estimated timing of cash flows	(4,511,619)		(4,561,364)		(3,048,103)
Standardized measure of discounted future net cash flows	3,833,415		4,035,279		2,799,156
BOE (average sales price in dollars)	71.74	[1]	71.18	[1]	52.93	[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	4,035,279		2,799,156		1,445,747
Sales of oil and natural gas produced, net of production costs	(625,707)		(599,679)		(406,390)
Net changes in sales prices and production costs	(786,022)		1,473,454		1,724,212
Revisions of previous quantity estimates	743		(281,765)		(49,784)
Change in estimated future development costs	(331,498)		(274,122)		(152,096)
Purchases of reserves in place	114,094		164,383		307,205
Sales of reserves in place	(15,283)		0		0
Development costs incurred during the period	497,036		433,660		144,086
Accretion of discount	570,505		383,418		184,917
Income taxes	323,128		(634,747)		(593,272)
Other	(73,326)		(29,792)		(112,513)
Net increase (decrease)	(201,864)		1,236,123		1,353,409
Standardized measure—end of year	3,833,415		4,035,279		2,799,156
Oil ($/Bbl)
Discounted Future Net Cash Flows Relating to Proved Oil and Natural Gas Reserves
Average sales prices (in dollars per production type)	90.66	[1]	93.72	[1]	69.04	[1]
Natural Gas ($/Mcf)
Discounted Future Net Cash Flows Relating to Proved Oil and Natural Gas Reserves
Average sales prices (in dollars per production type)	2.88	[1]	4.02	[1]	4.57	[1]
Improved Recovery
Increase (Decrease) in Standardized Measure of Discounted Future Net Cash Flow Relating to Proved Oil and Gas Reserves [Roll Forward]
Extensions, Discoveries, Additions and Improved Recovery, Less Related Costs	0		0		24,033
Extensions and Discoveries
Increase (Decrease) in Standardized Measure of Discounted Future Net Cash Flow Relating to Proved Oil and Gas Reserves [Roll Forward]
Extensions, Discoveries, Additions and Improved Recovery, Less Related Costs	$ 124,466		$ 601,313		$ 283,011

[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.