Net Income Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Numerator for Basic and Diluted Earnings per Share - [Abstract]
Net Income	$ 580,194	[1]	$ 462,498	[1]	$ 659,692	[1]	$ 494,725	[1]	$ (504,999)	[1]	$ 355,491	[1]	$ 395,778	[1]	$ 324,009	[1]	$ 2,197,109	$ 570,279	$ 1,091,123
Denominator for Basic Earnings per Share - [Abstract]
Weighted Average Shares (in shares)																	270,170,000	267,577,000	262,735,000
Potential Dilutive Common Shares -[Abstract]
Stock Options/SARs (in shares)																	1,159,000	1,456,000	1,707,000
Restricted Stock/Units and Performance Units/Stock (in shares)																	1,785,000	1,729,000	1,826,000
Denominator for Diluted Earnings per Share - [Abstract]
Adjusted Diluted Weighted Average Shares (in shares)	273,983,000		273,576,000		272,739,000		272,263,000		268,941,000		270,982,000		269,985,000		270,242,000		273,114,000	270,762,000	266,268,000
Net Income Per Share [Abstract]
Basic (in dollars per share)	$ 2.14	[2]	$ 1.71	[2]	$ 2.44	[2]	$ 1.84	[2]	$ (1.88)	[2]	$ 1.33	[2]	$ 1.48	[2]	$ 1.22	[2]	$ 8.13	$ 2.13	$ 4.15
Diluted (in dollars per share)	$ 2.12	[2]	$ 1.69	[2]	$ 2.42	[2]	$ 1.82	[2]	$ (1.88)	[2]	$ 1.31	[2]	$ 1.47	[2]	$ 1.20	[2]	$ 8.04	$ 2.11	$ 4.10
Antidilutive Stock Options and SARs excluded from Diluted Earnings Per Share Calculation (in shares)																	300,000	500,000	400,000

[1] The sum of quarterly net income (loss) per share may not agree with total year net income (loss) per share as each quarterly computation is based on the weighted average of common shares outstanding.

[2] Fourth quarter 2012 results include the impact of pretax impairments of $1,020 million, primarily related to proved and unproved natural gas properties in Canada and the United States as well as an additional income tax provision of $135 million related to valuation allowances recorded to reduce the value of Canadian deferred tax assets.