Fair Value Measurements and Credit Concentration (Nonrecurring Fair Value Measurements and Credit Concentration) (Details) (USD $)	Jun. 30, 2011	Jun. 30, 2011 Carrying Amount [Member]		Mar. 31, 2011 Carrying Amount [Member]		Jun. 30, 2010 Carrying Amount [Member]		Mar. 31, 2010 Carrying Amount [Member]		Jun. 30, 2011 Carrying Amount [Member]		Dec. 31, 2010 Carrying Amount [Member]		Jun. 30, 2011 Level 2 [Member]		Mar. 31, 2011 Level 2 [Member]		Mar. 31, 2011 Level 3 [Member]		Jun. 30, 2010 Level 3 [Member]		Mar. 31, 2010 Level 3 [Member]		Jun. 30, 2011 Loss [Member]		Mar. 31, 2011 Loss [Member]		Jun. 30, 2010 Loss [Member]		Mar. 31, 2010 Loss [Member]		Jun. 30, 2011 Fair Value [Member]		Dec. 31, 2010 Fair Value [Member]
Nonrecurring Fair Value Measurements
Sulfur dioxide emission allowances				$ 1,000,000	[1],[2]	$ 11,000,000	[1],[2]	$ 13,000,000	[1],[2]									$ 0	[1]	$ 3,000,000	[1]	$ 10,000,000	[1]			$ 1,000,000	[1],[3]	$ 8,000,000	[1],[3]	$ 3,000,000	[1],[3]
RECs		2,000,000	[1],[2]	3,000,000	[1],[2]									1,000,000	[1]	0	[1]							1,000,000	[1],[3]	3,000,000	[1],[3]
Fair value of Financial Instruments Not Recorded at Fair Value - Other
Contract adjustment payments										245,000,000	[4]	146,000,000	[4]																			247,000,000	[4]	148,000,000	[4]
Long-term debt										18,034,000,000		12,663,000,000																				18,447,000,000		12,868,000,000
Credit Concentration Associated with Financial Instruments (Numeric)
Gross credit exposure to energy trading partners	2,200,000,000
Net credit exposure to energy trading partners	670,000,000
Percentage exposure to single energy trading counterparty	13.00%
Percentage exposure to next highest energy trading counterparty	13.00%
Number of energy trading counterparties that have been aggregated	10
Dollar exposure to top ten energy trading counterparties	$ 441,000,000
Percentage exposure to top ten energy trading counterparties	66.00%
Number of counterparties in top ten that have investment grade credit rating	9
Percentage of top ten's exposure that consists of counterparties with investment grade credit rating (in hundreths)	97.00%

[1] Current and long-term sulfur dioxide emission allowances and RECs are included in "Other intangibles" in their respective areas on the Balance Sheets.

[2] Represents carrying value before fair value measurement.

[3] Losses on sulfur dioxide emission allowances and RECs were recorded in the Supply segment and included in "Other operation and maintenance" on the Statements of Income.

[4] Reflected in current and long-term other liabilities on the Balance Sheets.