Significant Accounting Policies (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Accrued unbilled revenue	$ 423,000,000	$ 466,000,000
Percentage of fuel currently subject to deferred fuel accounting	84.00%
Reduction in interest expense	7,000,000	18,000,000	19,000,000
Reduction in penalties	1,000,000	1,000,000	2,000,000
Accrued interest receivable	48,000,000	27,000,000
Interest and penalties payable	10,000,000	1,000,000
Income Tax Examination, Penalties Accrued	1,000,000
Accounts payable for checks outstanding	75,000,000	56,000,000
Margin assets	319,000,000	244,000,000
Margin liabilities	66,000,000	62,000,000
Capitalized interest costs	85,000,000	102,000,000	76,000,000
Ceiling test impairment charge, before tax		21,000,000	455,000,000
Ceiling test impairment charge, after tax		13,000,000	281,000,000
Ceiling test impairment excluding the effects of hedge-adjusted prices, before tax			631,000,000
Ceiling test impairment excluding the effects of hedge-adjusted prices, after tax			387,000,000
Inventory under LIFO method		48,000,000
Amount exceeded on LIFO basis	$ 86,000,000	$ 107,000,000