Fair Value Measurements and Credit Concentration (Nonrecurring Fair Value Measurements and Credit Concentration) (Details) (USD $)	Dec. 31, 2011	Jun. 30, 2009	Dec. 31, 2011 Carrying Amount [Member]	Dec. 31, 2010 Carrying Amount [Member]	Dec. 31, 2011 Fair Value [Member]	Dec. 31, 2010 Fair Value [Member]	Sep. 30, 2009 Nonrecurring [Member]	Sep. 30, 2011 Nonrecurring [Member] Carrying Amount [Member]		Jun. 30, 2011 Nonrecurring [Member] Carrying Amount [Member]		Mar. 31, 2011 Nonrecurring [Member] Carrying Amount [Member]		Dec. 31, 2010 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]		Dec. 31, 2009 Nonrecurring [Member] Carrying Amount [Member]		Sep. 30, 2009 Nonrecurring [Member] Carrying Amount [Member]		Jun. 30, 2009 Nonrecurring [Member] Carrying Amount [Member]		Mar. 31, 2009 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]	Dec. 31, 2010 Nonrecurring [Member] Level 2 [Member]	Sep. 30, 2010 Nonrecurring [Member] Level 2 [Member]	Jun. 30, 2010 Nonrecurring [Member] Level 2 [Member]	Mar. 31, 2010 Nonrecurring [Member] Level 2 [Member]	Dec. 31, 2009 Nonrecurring [Member] Level 2 [Member]	Sep. 30, 2009 Nonrecurring [Member] Level 2 [Member]	Jun. 30, 2009 Nonrecurring [Member] Level 2 [Member]	Mar. 31, 2009 Nonrecurring [Member] Level 2 [Member]	Sep. 30, 2011 Nonrecurring [Member] Level 3 [Member]	Jun. 30, 2011 Nonrecurring [Member] Level 3 [Member]	Mar. 31, 2011 Nonrecurring [Member] Level 3 [Member]	Dec. 31, 2010 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]		Dec. 31, 2009 Nonrecurring [Member] Level 3 [Member]		Sep. 30, 2009 Nonrecurring [Member] Level 3 [Member]	Jun. 30, 2009 Nonrecurring [Member] Level 3 [Member]	Mar. 31, 2009 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]		Dec. 31, 2010 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]		Dec. 31, 2009 Nonrecurring [Member] Loss [Member]		Sep. 30, 2009 Nonrecurring [Member] Loss [Member]		Jun. 30, 2009 Nonrecurring [Member] Loss [Member]		Mar. 31, 2009 Nonrecurring [Member] Loss [Member]
Nonrecurring Fair Value Measurements [Abstract]
Sulfur dioxide emission allowances								$ 1,000,000	[1],[2]			$ 1,000,000	[1],[2]	$ 2,000,000	[1],[2]	$ 6,000,000	[1],[2]	$ 11,000,000	[1],[2]	$ 13,000,000	[1],[2]	$ 20,000,000	[1],[2]					$ 45,000,000	[1],[2]	$ 0			$ 0	$ 0	$ 0	$ 0	$ 0	$ 0			$ 0	$ 0		$ 0	$ 1,000,000	[1]	$ 2,000,000	[1]	$ 3,000,000	[1]	$ 10,000,000	[1]	$ 13,000,000	[1]			$ 15,000,000	[1]	$ 1,000,000	[1],[3]			$ 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]	$ 7,000,000	[1],[3]					$ 30,000,000	[1],[3]
Certain non-core generation facilities																473,000,000	[2]																		381,000,000												0																				96,000,000	[3]
Long Island generation business																						132,000,000	[2]	137,000,000	[2]	189,000,000	[2]											128,000,000	133,000,000	138,000,000													0		0	0																	5,000,000	[3]	5,000,000	[3]	52,000,000	[3]
RECs								1,000,000	[1],[2]	2,000,000	[1],[2]	3,000,000	[1],[2]																	0	1,000,000	[1]	0									0	0	0															1,000,000	[1],[3]	1,000,000	[1],[3]	3,000,000	[1],[3]
Estimated costs to sell the Long Island generation business		1,000,000
Estimated costs to sell the non-core generation assets							4,000,000
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Contract adjustment payments			198,000,000	146,000,000	198,000,000	148,000,000
Long-term debt			17,993,000,000	12,663,000,000	19,392,000,000	12,868,000,000
Credit Concentration Associated with Financial Instruments (Numeric) [Abstract]
Gross credit exposure to energy trading partners	3,000,000,000
Net credit exposure to energy trading partners	866,000,000
Percentage exposure to single energy trading counterparty	11.00%
Percentage exposure to next highest energy trading counterparty	6.00%
Number of energy trading counterparties that have been aggregated	10
Dollar exposure to top ten energy trading counterparties	457,000,000
Percentage exposure to top ten energy trading counterparties	53.00%
Outstanding accounts receivable from a bankrupt counterparty	14,000,000
Reserve related to the accounts receivable from a bankrupt counterparty	$ 11,000,000

[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 and the Long Island generation business were recorded in the Supply segment and included in "Income (Loss) from Discontinued Operations (net of income taxes)" on the Statements of Income.