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Summary of Significant Accounting Policies and Other Matters - Investments (Details) $ in Millions | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2020 USD ($) mi | Dec. 30, 2020 USD ($) | Dec. 31, 2019 USD ($) | Jan. 31, 2016
Schedule of Equity Method Investments [Line Items] |  |  |  |
Supplemental retirement income plan assets | $ 465 |  | $ 397 |
Deferred income plan assets | 92 |  | 81 |
Other | 3 |  | 2 |
Total investments | $ 1,816 |  | 2,065 |
Stagecoach |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Equity method investment, ownership percentage | 50.00% |  |  |
Mountain Valley Pipeline |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Equity method investments | $ 530 |  | 530 |
Mountain Valley Pipeline | Minimum |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Aggregate project cost | 5,800 |  |  |
Mountain Valley Pipeline | Maximum |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Aggregate project cost | $ 6,000 |  |  |
Mountain Valley Pipeline | Mountain Valley Pipeline |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Expected ownership percentage | 8.80% |  |  |
CET | Stagecoach |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Equity method investments | $ 845 |  | 924 |
CET | Mountain Valley Pipeline |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Equity method investments | 342 |  | 602 |
CET | New York Transco, LLC |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Equity method investments | 69 |  | 59 |
CECONY |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Supplemental retirement income plan assets | 439 |  | 371 |
Deferred income plan assets | 92 |  | 81 |
Other | 10 |  | 9 |
Total investments | 541 |  | 461 |
CECONY | CET | Stagecoach |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Equity method investments | 0 |  | 0 |
CECONY | CET | Mountain Valley Pipeline |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Equity method investments | 0 |  | 0 |
CECONY | CET | New York Transco, LLC |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Equity method investments | 0 |  | 0 |
CET Gas | Mountain Valley Pipeline |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Equity method investments | $ 530 |  | $ 530 |
Equity method investment, ownership percentage | 11.30% |  |  |
Carrying value of investment | $ 342 | $ 662 |  |
Number of miles in pipeline construction | mi | 77 |  |  |
Long -term firm transportation contract period | 20 years |  |  |
Cash flows discount rate | 8.30% |  |  |
Impairment loss | $ 320 |  |  |
Expected reduction in equity interest | 8.80% |  |  |
Impairment loss, after tax | $ 223 |  |  |
CET Gas | Mountain Valley Pipeline | Mountain Valley Pipeline |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Equity method investment, ownership percentage |  |  |  | 12.50%

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