Acquisitions and Divestitures Acquisitions and Divestitures (Components of Discontinued Operations) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Oil and gas	$ 517	$ 890
Interest and other	6	5
Gain on disposition of assets, net	(1)	(6)
Oil and gas production	180	158
Production and ad valorem taxes	39	55
Depletion, depreciation and amortization	310	217
Impairment of oil and gas properties	138	0
Exploration and abandonment	25	31
General and administrative	82	81
Asset Retirement Obligation, Accretion Expense	3	3
Other Nonoperating Expense	48	15
Costs and Expenses	979	748
Current tax provision	0	(12)
Deferred tax benefit	37	(39)
Income (Loss) from Discontinued Operations, Net of Tax	(4)	(22)
Discontinued Operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total Revenues And Other Income From Discontinued Operations	0	118
Costs and Expenses	6	152
Loss from discontinued operations before income taxes	(6)	(34)
Deferred tax benefit	2	12
Income (Loss) from Discontinued Operations, Net of Tax	$ (4)	$ (22)