Stock-Based Compensation (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Stock based compensation by job function [Line Items]
Compensation expense related to the company's stock-based compensation plans	$ 30.4	$ 28.3
Common Shares Available for Grant	3,200,000
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized	15,500,000
Stock-based compensation expense related to stock options, SAR and ESPP grants	10.4	10.9
Share Based Compensation Arrangement By Share Based Payment Award Stock Option/SAR [Abstract]
Unrecognized compensation expense related to non-vested stock option and SAR grants	83
Weighted average period over which unrecognized compensation expense will be recognized (in years)	2.5
Performance Units and Performance Stock [Member]
Performance Units and Performance Stock [Abstract]
Performance Units Granted	54,526
Performance Stock Granted	16,752
Share Based Compensation Arrangement By Performance Units and Stock Compensation Cost	0.3
Employee Service Share Based Compensation Unrecognized Compensation Costs On Nonvested Performance Units and Stock Awards	3
Employee Service Share Based Compensation Unrecognized Compensation Costs On Nonvested Performance Units and Stock Awards Weighted Average Period Of Recognition	2.8
Stock Options And Sars [Member]
Stock Options/SARs and Employee Stock Purchase Plan (ESPP) Disclosures [Line Items]
Stock Options/SARs Exercisable at March 31 (in shares)	2,603,000	4,594,000
Stock Options/SARs Exercisable at March 31 (price per share)	$ 77.90	$ 59.20
Stock options/SARs vested or expected to vest (in shares)	5,410,000	7,456,000
Weighted average grant price for stock options/SARs vested or expected to vest (price per share)	$ 87.81	$ 69.98
Weighted Average Fair Value And Valuation Assumptions Used To Value Stock Options/SARs, ESPP, and Performance Units/Stock Stock Based Compensation [Abstract]
Weighted Average Fair Value of Grants	$ 38.61	$ 37.97
Expected Volatility (in hundredths)	35.54%	41.11%
Risk-Free Interest Rate (in hundredths)	0.50%	0.49%
Dividend Yield (in hundredths)	0.60%	0.60%
Expected Life (in years)	5.6	5.6
Stock Options and SARs Rollforward [Abstract]
Outstanding at January 1	6,219,000	8,374,000
Granted	15,000	20,000
Exercised (1)	(561,000)	(656,000)
Forfeited	(30,000)	(36,000)
Outstanding at March 31	5,643,000	7,702,000
Share Based Compensation Arrangement By Share Based Payment Award Stock Option/SAR [Abstract]
Outstanding at January 1 - Price per Share	$ 85.81	$ 70.01
Granted - Price Per Share	$ 125.04	$ 107.99
Exercised - Price Per Share	$ 62.32	$ 63.99
Forfeited - Price per Share	$ 94.50	$ 87.99
Outstanding at March 31 - Price per Share	$ 88.21	$ 70.54
Intrinsic value of stock options/SARs exercised during the period	36.2	32.4
Aggregate Intrinsic Value For Outstanding Options and SARs	225.1	313.3
Weighted Average Remaining Contractual Life for Stock Options/SARs Outstanding	4.2	3.5
Aggregate Intrinsic value of stock options/SARs vested or expected to vest	218.0	307.5
Weighted Average Remaining Contractual Life for Stock Options/SARs Vested or Expected to Vest	4.1	3.5
Aggregate Intrinsic Value For Exercisable Units	130.7	239.1
Weighted Average Remaining Contractual Life for Exercisable Options and SARs (in years)	2.7	2.2
Espp [Member]
Weighted Average Fair Value And Valuation Assumptions Used To Value Stock Options/SARs, ESPP, and Performance Units/Stock Stock Based Compensation [Abstract]
Weighted Average Fair Value of Grants	$ 28.80	$ 28.24
Expected Volatility (in hundredths)	29.95%	46.42%
Risk-Free Interest Rate (in hundredths)	0.12%	0.06%
Dividend Yield (in hundredths)	0.60%	0.60%
Expected Life (in years)	0.5	0.5
Restricted Stock And Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award Stock Option/SAR [Abstract]
Share Based Compensation Arrangement By Restricted Stock And Restricted Stock Units Compensation Cost	19.7	17.4
Restricted Stock and Restricted Stock Unit [Roll Forward]
Outstanding at January 1	3,818,000	4,240,000
Granted	250,000	271,000
Released (1)	(153,000)	(466,000)
Forfeited	(31,000)	(36,000)
Outstanding at March 31 (2)	3,884,000	4,009,000
Weighted Average Grant Price Restricted Stock and Restricted Stock Units Rollforward [Abstract]
Outstanding at January 1 - Price per Share	$ 91.06	$ 82.93
Granted - Price per Share	$ 128.57	$ 112.53
Released - Price per Share	$ 119.62	$ 70.13
Forfeited - Price per Share	$ 93.32	$ 88.18
Outstanding at March 31 - Price per Share	$ 92.33	$ 86.36
Intrinsic value of restricted stock and restricted stock units released during the year	19.0	53.1
Aggregate Intrinsic Value Of Restricted Stock And Restricted Stock Units Outstanding	497.4	445.4
Employee Service Share Based Compensation Unrecognized Compensation Costs On Nonvested Restricted Stock Awards	146
Employee Service Share Based Compensation Unrecognized Compensation Costs On Nonvested Restricted Stock Awards Weighted Average Period Of Recognition	2.6
Lease And Well [Member]
Stock based compensation by job function [Line Items]
Compensation expense related to the company's stock-based compensation plans	9.8	8.5
Gathering And Processing Costs [Member]
Stock based compensation by job function [Line Items]
Compensation expense related to the company's stock-based compensation plans	0.3	0.2
Exploration Costs [Member]
Stock based compensation by job function [Line Items]
Compensation expense related to the company's stock-based compensation plans	7.5	6.6
General And Administrative [Member]
Stock based compensation by job function [Line Items]
Compensation expense related to the company's stock-based compensation plans	$ 12.8	$ 13.0