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Regulatory Matters - Summary of Utilities Rate Plans (CECONY-Steam) (Details) - CECONY - Steam - USD ($) | 12 Months Ended | 36 Months Ended |
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2016 | Dec. 31, 2014
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |
Utilities operating expense, depreciation and amortization |  |  |  |  |  |  | $ 37,000,000 |
Regulatory liabilities, amortization period |  |  |  |  |  |  | 3 years |
Negative revenue adjustments | $ 0 | $ 0 | $ 0 | $ 0 | $ 0 | $ 0 |  |
Cost reconciliation, deferred net regulatory liabilities |  |  |  | $ 14,000,000 | 8,000,000 |  | $ 8,000,000 | $ 42,000,000
Cost reconciliation, deferred net regulatory assets | 35,000,000 | 8,000,000 | 1,000,000 |  |  | 17,000,000 |  |
Increase (decrease) in regulatory liabilities | 0 | 0 | 0 |  |  | (100,000) |  |
Authorized return on common equity (percent) |  |  |  |  |  |  | 930.00% |
Earnings sharing (percent) |  |  |  | 9.90% |  |  |  |
Earnings sharing, threshold limit | $ 5,000,000 | 16,500,000 | 8,500,000 | $ 7,800,000 | $ 11,500,000 | $ 0 |  |
Earnings sharing, positive adjustment |  | $ 2,300,000 | $ 1,100,000 | 4,000,000 |  |  |  |
Common Equity Ratio, Percentage |  |  |  |  |  |  | 4800.00% |
Other regulatory liabilities |  |  |  | $ 8,000,000 |  |  |  |
Difference in property taxes (percent) |  |  |  |  |  |  | 90.00% |
Deferral, annual maximum (not more than) (percent) |  |  |  |  |  |  | 10.00% |
Year 1 |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |
Base rate changes |  |  |  |  |  |  | $ (22,400,000) |
Average rate base |  |  |  |  |  |  | $ 1,511,000,000 |
Weighted average cost of capital (after-tax) (percent) |  |  |  |  |  |  | 7.10% |
Actual return on common equity (percent) |  |  |  |  |  |  | 9.82% |
Cost of long-term debt rate |  |  |  | 5.17% |  |  |  |
Year 1 | Production |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |
Net utility plant reconciliations |  |  |  |  |  |  | $ 1,752,000,000 |
Year 1 | Distribution |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |
Net utility plant reconciliations |  |  |  |  |  |  | 6,000,000 |
Year 1 | Maximum |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |
Potential penalties (annually) |  |  |  |  |  |  | 1,000,000 |
Year 2 |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |
Base rate changes |  |  |  |  |  |  | 19,800,000 |
Average rate base |  |  |  |  |  |  | $ 1,547,000,000 |
Weighted average cost of capital (after-tax) (percent) |  |  |  |  |  |  | 7.13% |
Actual return on common equity (percent) |  |  |  |  |  |  | 10.88% |
Cost of long-term debt rate |  |  |  | 5.23% |  |  |  |
Year 2 | Production |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |
Net utility plant reconciliations |  |  |  |  |  |  | $ 1,732,000,000 |
Year 2 | Distribution |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |
Net utility plant reconciliations |  |  |  |  |  |  | 11,000,000 |
Year 2 | Maximum |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |
Potential penalties (annually) |  |  |  |  |  |  | 1,000,000 |
Year 3 |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |
Base rate changes |  |  |  |  |  |  | 20,300,000 |
Average rate base |  |  |  |  |  |  | $ 1,604,000,000 |
Weighted average cost of capital (after-tax) (percent) |  |  |  |  |  |  | 7.21% |
Actual return on common equity (percent) |  |  |  |  |  |  | 10.54% |
Cost of long-term debt rate |  |  |  | 5.39% |  |  |  |
Year 3 | Production |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |
Net utility plant reconciliations |  |  |  |  |  |  | $ 1,720,000,000 |
Year 3 | Distribution |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |
Net utility plant reconciliations |  |  |  |  |  |  | 25,000,000 |
Year 3 | Maximum |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |
Potential penalties (annually) |  |  |  |  |  |  | 1,000,000 |
Year 4 |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |
Base rate changes |  |  |  |  |  |  | $ 0 |
Actual return on common equity (percent) |  |  |  |  |  |  | 9.51% |
Year 5 |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |
Base rate changes |  |  |  |  |  |  | $ 0 |
Actual return on common equity (percent) |  |  |  |  |  |  | 11.73% |
Year 6 |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |
Base rate changes |  |  |  |  |  |  | $ 0 |
Actual return on common equity (percent) |  |  |  |  |  |  | 10.45% |
Year 7 |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |
Actual return on common equity (percent) |  |  |  |  |  |  | 7.91% |

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