Fair Value Measurements and Credit Concentration (Nonrecurring Fair Value Measurements and Credit Concentration) (Details) (USD $)
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Dec. 31, 2011
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Jun. 30, 2009
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Dec. 31, 2011
Carrying Amount [Member]
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Dec. 31, 2010
Carrying Amount [Member]
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Dec. 31, 2011
Fair Value [Member]
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Dec. 31, 2010
Fair Value [Member]
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Sep. 30, 2009
Nonrecurring [Member]
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Sep. 30, 2011
Nonrecurring [Member]
Carrying Amount [Member]
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Jun. 30, 2011
Nonrecurring [Member]
Carrying Amount [Member]
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Mar. 31, 2011
Nonrecurring [Member]
Carrying Amount [Member]
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Dec. 31, 2010
Nonrecurring [Member]
Carrying Amount [Member]
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Sep. 30, 2010
Nonrecurring [Member]
Carrying Amount [Member]
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Jun. 30, 2010
Nonrecurring [Member]
Carrying Amount [Member]
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Mar. 31, 2010
Nonrecurring [Member]
Carrying Amount [Member]
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Dec. 31, 2009
Nonrecurring [Member]
Carrying Amount [Member]
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Sep. 30, 2009
Nonrecurring [Member]
Carrying Amount [Member]
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Jun. 30, 2009
Nonrecurring [Member]
Carrying Amount [Member]
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Mar. 31, 2009
Nonrecurring [Member]
Carrying Amount [Member]
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Sep. 30, 2011
Nonrecurring [Member]
Level 2 [Member]
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Jun. 30, 2011
Nonrecurring [Member]
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Mar. 31, 2011
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Dec. 31, 2010
Nonrecurring [Member]
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Sep. 30, 2010
Nonrecurring [Member]
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Jun. 30, 2010
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Mar. 31, 2010
Nonrecurring [Member]
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Dec. 31, 2009
Nonrecurring [Member]
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Sep. 30, 2009
Nonrecurring [Member]
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Jun. 30, 2009
Nonrecurring [Member]
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Mar. 31, 2009
Nonrecurring [Member]
Level 2 [Member]
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Sep. 30, 2011
Nonrecurring [Member]
Level 3 [Member]
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Jun. 30, 2011
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Level 3 [Member]
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Mar. 31, 2011
Nonrecurring [Member]
Level 3 [Member]
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Dec. 31, 2010
Nonrecurring [Member]
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Sep. 30, 2010
Nonrecurring [Member]
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Jun. 30, 2010
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Level 3 [Member]
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Mar. 31, 2010
Nonrecurring [Member]
Level 3 [Member]
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Dec. 31, 2009
Nonrecurring [Member]
Level 3 [Member]
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Sep. 30, 2009
Nonrecurring [Member]
Level 3 [Member]
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Jun. 30, 2009
Nonrecurring [Member]
Level 3 [Member]
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Mar. 31, 2009
Nonrecurring [Member]
Level 3 [Member]
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Sep. 30, 2011
Nonrecurring [Member]
Loss [Member]
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Jun. 30, 2011
Nonrecurring [Member]
Loss [Member]
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Mar. 31, 2011
Nonrecurring [Member]
Loss [Member]
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Dec. 31, 2010
Nonrecurring [Member]
Loss [Member]
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Sep. 30, 2010
Nonrecurring [Member]
Loss [Member]
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Jun. 30, 2010
Nonrecurring [Member]
Loss [Member]
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Mar. 31, 2010
Nonrecurring [Member]
Loss [Member]
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Dec. 31, 2009
Nonrecurring [Member]
Loss [Member]
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Sep. 30, 2009
Nonrecurring [Member]
Loss [Member]
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Jun. 30, 2009
Nonrecurring [Member]
Loss [Member]
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Mar. 31, 2009
Nonrecurring [Member]
Loss [Member]
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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 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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