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Asset Impairments (Details 1) $ in Millions |  |  |  | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | --- | --- | ---
Oct. 20, 2017 | Aug. 30, 2017 | Aug. 30, 2017 MW | Aug. 30, 2017 Natural_gas_plant | Aug. 30, 2017 transmission_center | Aug. 30, 2017 USD ($) | Aug. 30, 2017 | Aug. 30, 2017 limited_guaranty | Jul. 22, 2016 MW | Jan. 31, 2017 USD ($) MW | Sep. 30, 2017 USD ($) | Sep. 30, 2016 USD ($) | Jun. 30, 2016 USD ($) | Sep. 30, 2017 USD ($) | Sep. 30, 2016 USD ($) | Dec. 31, 2016 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Impairment of assets |  |  |  |  |  |  |  |  |  |  |  |  | $ 647 |  |  |
Impairment of assets (Note 14) |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 162 | $ 1,447 |
Impairment |  |  |  |  |  |  |  |  |  |  |  |  | 800 |  |  | $ 800
Contract Termination |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restructuring charges |  |  |  |  |  |  |  |  |  |  |  |  | $ 58 |  |  |
Competitive Energy Services |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Impairment of assets |  |  |  |  |  |  |  |  |  |  |  |  |  | 158 |  |
Competitive Energy Services | Income Approach Valuation Technique |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Discount rate (percent) |  |  |  |  |  |  |  |  |  |  |  |  | 9.50% |  |  |
Terminal value of EBITDA |  |  |  |  |  |  |  |  |  |  |  |  | 7.0 |  |  |
FES |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Impairment of assets |  |  |  |  |  |  |  |  |  |  |  |  | $ 517 |  |  |
Impairment of assets (Note 14) |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 | $ 540 |
Impairment |  |  |  |  |  |  |  |  |  |  |  |  | $ 23 |  |  |
Loss on contract termination |  |  |  |  |  |  |  |  |  |  |  | $ 32 |  |  |  |
MAIT |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Impairment of assets (Note 14) |  |  |  |  |  |  |  |  |  |  | $ 13 |  |  |  |  |
Purchase Agreement with Aspen Generating, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Plant capacity (in MW's) | MW |  |  | 1,615 |  |  |  |  |  |  | 1,572 |  |  |  |  |  |
Cash purchase price |  |  |  |  |  | $ 825 |  |  |  | $ 925 |  |  |  |  |  |
Number of limited guaranties |  |  |  |  | 2 |  |  | 2 |  |  |  |  |  |  |  |
Term of guaranties |  | 3 years |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase Agreement with Aspen Generating, LLC | AE Supply |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of gas generating plants | Natural_gas_plant |  |  |  | 4 |  |  |  |  |  |  |  |  |  |  |  |
Discharge of note indenture |  |  |  |  |  | 305 |  |  |  |  |  |  |  |  |  |
Make-whole premiums |  |  |  |  |  | $ 100 |  |  |  |  |  |  |  |  |  |
Purchase Agreement with Aspen Generating, LLC | AGC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership percentage |  |  |  |  |  |  | 59.00% |  |  |  |  |  |  |  |  |
Purchase Agreement with Aspen Generating, LLC | AGC | Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership percentage | 59.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase Agreement with Aspen Generating, LLC | BU Energy |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership percentage |  |  |  |  |  |  | 50.00% |  |  |  |  |  |  |  |  |
Purchase Agreement with Aspen Generating, LLC | Buchanan Power Station [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Plant capacity (in MW's) | MW |  |  | 43 |  |  |  |  |  |  |  |  |  |  |  |  |
Disposal Group | Bay Shore Unit 1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Plant capacity (in MW's) | MW |  |  |  |  |  |  |  |  | 136 |  |  |  |  |  |  |
Disposal Group | Sammis Power Plant Units 1-4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Plant capacity (in MW's) | MW |  |  |  |  |  |  |  |  | 720 |  |  |  |  |  |  |
Held-for-sale |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Assets held-for-sale |  |  |  |  |  |  |  |  |  |  | 765 |  |  | 765 |  |
Investments |  |  |  |  |  |  |  |  |  |  | 20 |  |  | 20 |  |
Inventory |  |  |  |  |  |  |  |  |  |  | 4 |  |  | 4 |  |
Asset retirement obligations |  |  |  |  |  |  |  |  |  |  | $ 1 |  |  | $ 1 |  |

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