Acquisitions and Divestitures (Details) (USD $)	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011 acre	Dec. 31, 2010
Discontinued Operation, Income (Loss) from Discontinued Operation Disclosures [Abstract]
Oil and Gas Sales	$ 208,000,000		$ 357,000,000	$ 309,000,000
Exploration Expense - Selkirk	27,000,000
Income Before Income Taxes	101,000,000		215,000,000	183,000,000
Income Tax Expense	55,000,000		174,000,000	89,000,000
Operating Income, Net of Tax	46,000,000		41,000,000	94,000,000
Gain on Sale, Net of Tax	16,000,000		0	0
Discontinued Operations, Net of Tax	62,000,000		41,000,000	94,000,000
North Sea Divestiture [Abstract]
Interest in Dumbarton and Lochranza Fields	30.00%
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash Proceeds	1,160,000,000		77,000,000	564,000,000
Goodwill Allocated to Assets Sold	(61,000,000)	[1]	0
Gain on Divestitures	154,000,000		25,000,000	113,000,000
Marcellus Shale Joint Venture [Abstract]
CONSOL JV, Ownership Percentage			50.00%
Acreage Acquired (acres)			628,000
Payments to Date to Acquire Interest in CONSOL JV	938,000,000
Ratio of funding on share of future drilling and completion costs			one-third
Maximum amount to be paid each calendar year for funding of future drilling and completion costs			400,000,000
Funding of joint venture's future drilling and completion costs			2,100,000,000
Natural gas price agreed upon benchmark, average			4.00
Imputed interest on CONSOL Installment Payments	17,000,000
Total Purchase Price [Abstract]
Unproved Oil and Gas Properties	803,000,000
Proved Oil and Gas Properties	386,000,000
Investment in CONE Gathering LLC	69,000,000
Total Assets Acquired	1,258,000,000	[2]	1,300,000,000
US Onshore Assets Sold [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash Proceeds	1,044,000,000			552,000,000
Net Book Value of Assets Sold	(836,000,000)			(394,000,000)
Goodwill Allocated to Assets Sold	(61,000,000)			(61,000,000)
Asset Retirement Obligations Associated with Assets Sold	20,000,000			10,000,000
Other Closing Adjustments	(13,000,000)			3,000,000
Gain on Divestitures	(154,000,000)			(110,000,000)
North Sea Assets Sold [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash Proceeds	117,000,000
Net Book Value of Assets Sold	255,000,000
Deferred Tax Liability Release	99,000,000
Asset Retirement Obligations Associated with Assets Sold	$ 55,000,000

[1] See Note 3. Acquisitions and Divestitures.

[2] Total reflects impact of $17 million imputed interest on CONSOL installment payments.