Fair Value Measurements and Credit Concentration (Nonrecurring Fair Value Measurements and Credit Concentration) (Details) (USD $)	Sep. 30, 2011	Sep. 30, 2011 Nonrecurring [Member] Carrying Amount [Member]		Jun. 30, 2011 Nonrecurring [Member] Carrying Amount [Member]		Mar. 31, 2011 Nonrecurring [Member] Carrying Amount [Member]		Sep. 30, 2010 Nonrecurring [Member] Carrying Amount [Member]		Jun. 30, 2010 Nonrecurring [Member] Carrying Amount [Member]		Mar. 31, 2010 Nonrecurring [Member] Carrying Amount [Member]		Sep. 30, 2011 Nonrecurring [Member] Level 2 [Member]	Jun. 30, 2011 Nonrecurring [Member] Level 2 [Member]		Mar. 31, 2011 Nonrecurring [Member] Level 2 [Member]	Sep. 30, 2010 Nonrecurring [Member] Level 2 [Member]	Sep. 30, 2011 Nonrecurring [Member] Level 3 [Member]	Mar. 31, 2011 Nonrecurring [Member] Level 3 [Member]	Sep. 30, 2010 Nonrecurring [Member] Level 3 [Member]		Jun. 30, 2010 Nonrecurring [Member] Level 3 [Member]		Mar. 31, 2010 Nonrecurring [Member] Level 3 [Member]		Sep. 30, 2011 Nonrecurring [Member] Loss [Member]		Jun. 30, 2011 Nonrecurring [Member] Loss [Member]		Mar. 31, 2011 Nonrecurring [Member] Loss [Member]		Sep. 30, 2010 Nonrecurring [Member] Loss [Member]		Jun. 30, 2010 Nonrecurring [Member] Loss [Member]		Mar. 31, 2010 Nonrecurring [Member] Loss [Member]		Sep. 30, 2011 Carrying Amount [Member]		Dec. 31, 2010 Carrying Amount [Member]		Sep. 30, 2011 Fair Value [Member]		Dec. 31, 2010 Fair Value [Member]
Nonrecurring Fair Value Measurements [Abstract]
Sulfur dioxide emission allowances		$ 1,000,000	[1],[2]			$ 1,000,000	[1],[2]	$ 6,000,000	[1],[2]	$ 11,000,000	[1],[2]	$ 13,000,000	[1],[2]						$ 0	$ 0	$ 2,000,000	[1]	$ 3,000,000	[1]	$ 10,000,000	[1]	$ 1,000,000	[1],[3]			$ 1,000,000	[1],[3]	$ 4,000,000	[1],[3]	$ 8,000,000	[1],[3]	$ 3,000,000	[1],[3]
Certain non-core generation facilities								473,000,000	[2]									381,000,000															96,000,000	[3]
RECs		1,000,000	[1],[2]	2,000,000	[1],[2]	3,000,000	[1],[2]							0	1,000,000	[1]	0										1,000,000	[1],[3]	1,000,000	[1],[3]	3,000,000	[1],[3]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Contract adjustment payments																																							220,000,000	[4]	146,000,000	[4]	220,000,000	[4]	148,000,000	[4]
Long-term debt																																							18,177,000,000		12,663,000,000		19,369,000,000		12,868,000,000
Credit Concentration Associated with Financial Instruments (Numeric) [Abstract]
Gross credit exposure to energy trading partners	2,100,000,000
Net credit exposure to energy trading partners	766,000,000
Percentage exposure to single energy trading counterparty	15.00%
Percentage exposure to next highest energy trading counterparty	12.00%
Number of energy trading counterparties that have been aggregated	10
Dollar exposure to top ten energy trading counterparties	$ 519,000,000
Percentage exposure to top ten energy trading counterparties	68.00%
Number of counterparties in top ten that have investment grade credit rating	10
Percentage of top ten's exposure that consists of counterparties with investment grade credit rating (in hundreths)	68.00%

[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. Losses on certain non-core generation facilities were recorded in the Supply segment and included in "Income (Loss) from Discontinued Operations (net of income taxes)" on the Statements of Income.

[4] Included in "Other current liabilities" and "Other deferred credits and noncurrent liabilities" on the Balance Sheets.