Stock-Based Compensation (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011 Y
Stock based compensation by job function [Line Items]
Compensation expense related to the company's stock-based compensation plans	$ 27,100,000	$ 26,000,000	$ 55,400,000	$ 53,400,000
Common Shares Available for Grant	5,000,000		5,000,000
Stock-based compensation expense related to stock options, SAR and ESPP grants	10,500,000	10,500,000	21,300,000	19,900,000
Summary Information By Grant Price Range For Stock Options And SARs Outstanding And Exercisable At End Of Period [Line Items]
Aggregate Intrinsic Value For Outstanding Options and SARs	147,800,000	283,600,000	147,800,000	283,600,000
Aggregate Intrinsic Value For Exercisable Units	134,300,000	235,500,000	134,300,000	235,500,000
Weighted Average Remaining Contractual Life for Exercisable Options and SARs (in years)			2.0	2.4
Unrecognized compensation expense related to non-vested stock option and SAR grants	70,500,000		70,500,000
Weighted average period over which unrecognized compensation expense will be recognized (in years)			2.4
Share Based Compensation Arrangement By Share Based Payment Award Stock Option/SAR [Abstract]
Weighted Average Remaining Contractual Life for Stock Options/SARs Vested or Expected to Vest			3.3	3.5
Weighted Average Remaining Contractual Life for Stock Options/SARs Outstanding			3.4	3.6
Intrinsic value of stock options/SARs exercised during the period			45,400,000	59,900,000
Intrinsic value of stock options/SARs vested or expected to vest	146,700,000	279,800,000	146,700,000	279,800,000
Restricted Stock and Restricted Stock Unit [Roll Forward]
Employee Service Share Based Compensation Unrecognized Compensation Costs On Nonvested Restricted Stock Awards	124,900,000		124,900,000
Employee Service Share Based Compensation Unrecognized Compensation Costs On Nonvested Restricted Stock Awards Weighted Average Period Of Recognition			2.4
Weighted Average Grant Price Restricted Stock and Restricted Stock Units Rollforward [Abstract]
Share Based Compensation Arrangement By Restricted Stock And Restricted Stock Units Compensation Cost	16,600,000	15,500,000	34,100,000	33,500,000
Intrinsic value of restricted stock and restricted stock units released during the year	$ 55,700,000	$ 22,600,000	$ 55,700,000	$ 22,600,000
Aggregate Intrinsic Value Of Restricted Stock And Restricted Stock Units	357,300,000	417,300,000	357,300,000	417,300,000
Stock Options And Sars [Member]
Stock Options/SARs and Employee Stock Purchase Plan (ESPP) Disclosures [Line Items]
Stock Options/SARs Exercisable at June 30 (in shares)	4,379,000	4,510,000	4,379,000	4,510,000
Stock Options/SARs Exercisable at June 30 (price per share)	$ 60.20	$ 52.53	$ 60.20	$ 52.53
Stock options/SARs vested or expected to vest (in shares)	7,179,000	7,183,000	7,179,000	7,183,000
Weighted average grant price for stock options/SARs vested or expected to vest (per share)	$ 70.69	$ 65.84	$ 70.69	$ 65.84
Weighted Average Fair Value And Valuation Assumptions Used To Value Stock Options/SARs and ESPP Stock Based Compensation [Abstract]
Weighted Average Fair Value of Grants			$ 35.65	$ 36.57
Expected Volatility (in hundredths)			39.97%	37.13%
Risk-Free Interest Rate (in hundredths)			0.49%	1.12%
Dividend Yield (in hundredths)			0.70%	0.60%
Expected Life (in years)			5.5	5.4
Stock Options and SARs Rollforward [Abstract]
Outstanding at January 1			8,374,000	8,445,000
Granted			46,000	80,000
Exercised (1)			(920,000)	(1,016,000)
Forfeited			(82,000)	(99,000)
Outstanding at June 30	7,418,000	7,410,000	7,418,000	7,410,000
Share Based Compensation Arrangement By Share Based Payment Award Stock Option/SAR [Abstract]
Outstanding at January 1 - Price per Share			$ 70.01	$ 64.49
Granted - Price Per Share			$ 106.00	$ 110.36
Exercised - Price Per Share			$ 60.34	$ 51.11
Forfeited - Price per Share			$ 88.85	$ 87.22
Outstanding at June 30 - Price per Share	$ 71.23	$ 66.51	$ 71.23	$ 66.51
Espp [Member]
Weighted Average Fair Value And Valuation Assumptions Used To Value Stock Options/SARs and ESPP Stock Based Compensation [Abstract]
Weighted Average Fair Value of Grants			$ 28.24	$ 21.55
Expected Volatility (in hundredths)			46.42%	30.26%
Risk-Free Interest Rate (in hundredths)			0.06%	0.18%
Dividend Yield (in hundredths)			0.60%	0.60%
Expected Life (in years)			0.5	0.5
Restricted Stock And Restricted Stock Units [Member]
Restricted Stock and Restricted Stock Unit [Roll Forward]
Outstanding at January 1			4,240,000	4,009,000
Granted			290,000	292,000
Released (1)			(490,000)	(213,000)
Forfeited			(75,000)	(97,000)
Outstanding at June 30 (2)	3,965,000	3,991,000	3,965,000	3,991,000
Weighted Average Grant Price Restricted Stock and Restricted Stock Units Rollforward [Abstract]
Outstanding at January 1 - Price (per share)			82.93	79.13
Granted - Price (per share)			112.08	106.14
Released - Price (per share)			70.97	69.29
Forfeited - Price (per share)	$ 88.78	$ 80.09	$ 88.78	$ 80.09
Outstanding at June 30 - Price (per share)	86.42	81.61	86.42	81.61
Lease And Well [Member]
Stock based compensation by job function [Line Items]
Compensation expense related to the company's stock-based compensation plans	8,000,000	7,200,000	16,500,000	14,900,000
Gathering And Processing Costs [Member]
Stock based compensation by job function [Line Items]
Compensation expense related to the company's stock-based compensation plans	300,000	200,000	500,000	400,000
Exploration Costs [Member]
Stock based compensation by job function [Line Items]
Compensation expense related to the company's stock-based compensation plans	6,300,000	5,500,000	12,900,000	11,600,000
General And Administrative [Member]
Stock based compensation by job function [Line Items]
Compensation expense related to the company's stock-based compensation plans	$ 12,500,000	$ 13,100,000	$ 25,500,000	$ 26,500,000