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Organization and Basis of Presentation (Details Textuals) customer in Millions, MWh in Millions |  | 1 Months Ended | 2 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Aug. 30, 2017 | Aug. 30, 2017 MW | Aug. 30, 2017 Natural_gas_plant | Aug. 30, 2017 transmission_center | Aug. 30, 2017 USD ($) | Aug. 30, 2017 | Aug. 30, 2017 limited_guaranty | Dec. 31, 2017 USD ($) | Mar. 31, 2017 MWh | Jan. 31, 2017 USD ($) MW | Mar. 06, 2017 USD ($) MWh MW | Mar. 31, 2018 USD ($) | Sep. 30, 2017 USD ($) transmission_center MW | Sep. 30, 2016 USD ($) | Sep. 30, 2017 USD ($) customer transmission_center company mi MW | Sep. 30, 2016 USD ($) | Dec. 31, 2018 MWh | Dec. 31, 2017 MWh | Dec. 31, 2016 USD ($) MWh | Dec. 31, 2015 USD ($) MW | Dec. 31, 2020 MW | Mar. 07, 2017 MW
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate amount of capacity | MW |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 17,000 |  |  |  |  |  |  |
Length of transmission lines | mi |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 24,500 |  |  |  |  |  |  |
Number of regional transmission centers | transmission_center |  |  |  |  |  |  |  |  |  |  |  |  | 2 |  | 2 |  |  |  |  |  |  |
Capitalized cost of equity |  |  |  |  |  |  |  |  |  |  |  |  | $ 8,000,000 | $ 11,000,000 | $ 25,000,000 | $ 28,000,000 |  |  |  |  |  |
Capitalized interest |  |  |  |  |  |  |  |  |  |  |  |  | $ 11,000,000 | 17,000,000 | 34,000,000 | 51,000,000 |  |  |  |  |  |
Net cash used in operating activities |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (2,762,000,000) | (2,592,000,000) |  |  |  |  |  |
Net cash provided by financing activities |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ (381,000,000) | 304,000,000 |  |  |  |  |  |
Utilization of Accelerated Useful Life |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reduction to depreciation |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 21,000,000 |  |  |  |  |  | $ 19,000,000 |  |
Accounting Standards Update 2016-09 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net cash used in operating activities |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 12,000,000 |  |  |
Net cash provided by financing activities |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 12,000,000 |  |  |
Accounting Standards Update 2016-09 | Retained Earnings |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative effect of new accounting principle |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 6,000,000 |  |  |
Regulated Distribution |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of existing utility operating companies | company |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10 |  |  |  |  |  |  |
Number of customers served by utility operating companies | customer |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 6 |  |  |  |  |  |  |
Plant capacity (in MW's) | MW |  |  |  |  |  |  |  |  |  |  |  |  | 3,790 |  | 3,790 |  |  |  |  |  |  |
CES |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Capacity in deactivated power plants (in MW's) | MW |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 6,770 |  |
Plant capacity (in MW's) | MW |  |  |  |  |  |  |  |  |  |  | 13,000 |  | 13,162 |  | 13,162 |  |  |  |  |  |  | 10,000
Purchase power agreements additional capacity (in MWh) | MWh |  |  |  |  |  |  |  |  |  |  | 5 |  |  |  |  |  |  |  |  |  |  |
Contract sales (in MWh) | MWh |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 53 |  |  |
FES |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net cash used in operating activities |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ (458,000,000) | (605,000,000) |  |  |  |  |  |
Net cash provided by financing activities |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (83,000,000) | $ 61,000,000 |  |  |  |  |  |
FES | CES |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Inability to extend or refinance debt next year |  |  |  |  |  |  |  |  |  |  |  |  | $ 515,000,000 |  | 515,000,000 |  |  |  |  |  |  |
Scenario, Forecast | CES |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Capacity in deactivated power plants (in MW's) | MW |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 856 |
Scenario, Forecast | FES | CES |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest and lease payments |  |  |  |  |  |  |  | $ 48,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest payments |  |  |  |  |  |  |  |  |  |  |  | $ 38,000,000 |  |  |  |  |  |  |  |  |  |
Minimum | CES |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual generation capacity (in MW) | MWh |  |  |  |  |  |  |  |  |  |  | 70 |  |  |  |  |  |  |  |  |  |  |
Minimum | Scenario, Forecast | CES |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Expected contract sales (in MWh) | MWh |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 30 | 40 |  |  |  |
Maximum | CES |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual generation capacity (in MW) | MWh |  |  |  |  |  |  |  |  | 65 |  | 75 |  |  |  |  |  |  |  |  |  |  |
Maximum | Scenario, Forecast | CES |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Expected contract sales (in MWh) | MWh |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 35 | 45 |  |  |  |
Line of Credit | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum amount borrowed under revolving credit facility |  |  |  |  |  |  |  |  |  |  |  |  | $ 500,000,000 |  | $ 500,000,000 |  |  |  |  |  |  |
Purchase Agreement with Aspen Generating, LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Plant capacity (in MW's) | MW |  | 1,615 |  |  |  |  |  |  |  | 1,572 |  |  |  |  |  |  |  |  |  |  |  |
Cash purchase price |  |  |  |  | $ 825,000,000 |  |  |  |  | $ 925,000,000 |  |  |  |  |  |  |  |  |  |  |  |
Number of limited guaranties |  |  |  | 2 |  |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Term of guaranties | 3 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase Agreement with Aspen Generating, LLC | AE Supply |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of gas generating plants | Natural_gas_plant |  |  | 4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Discharge of note indenture |  |  |  |  | 305,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Make-whole premiums |  |  |  |  | $ 100,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase Agreement with Aspen Generating, LLC | AGC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership percentage |  |  |  |  |  | 59.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase Agreement with Aspen Generating, LLC | BU Energy |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership percentage |  |  |  |  |  | 50.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Pleasants Power Station |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Plant capacity (in MW's) | MW |  |  |  |  |  |  |  |  |  |  | 1,300 |  |  |  |  |  |  |  |  |  |  |
Pleasants Power Station | CES |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Plant capacity (in MW's) | MW |  |  |  |  |  |  |  |  |  |  |  |  | 1,300 |  | 1,300 |  |  |  |  |  |  |
Held-for-sale | Pleasants Power Station |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Assets held for sale |  |  |  |  |  |  |  |  |  |  | $ 195,000,000 |  |  |  |  |  |  |  |  |  |  |

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