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Other Commitments And Contingencies (Narrative) (Details) | 3 Months Ended | 4 Months Ended | 12 Months Ended | 16 Months Ended | 28 Months Ended |
--- | --- | --- | --- | --- | --- | ---
Jun. 30, 2019 USD ($) | Mar. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2020 USD ($) Rate MW | Dec. 31, 2018 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2020 USD ($) Rate MW | Jul. 26, 2019 USD ($)
Other Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |
Payments made in connection with operating leases |  |  |  |  | $ 49,100,000 |  |  |
Wind power purchase agreement capacity | MW |  |  |  | 500 |  |  | 500 |
Major Rail Operators |  |  |  | 3 |  |  | 3 |
Line of Credit Facility, Amount Outstanding |  |  |  | $ 0 |  | $ 0 | $ 0 |
Proposed Class Action Settlement |  |  |  |  |  |  |  | $ 143,000,000
Expenses Related to Third-Party Claims, Fines, Penalties, and settlements |  |  |  | (5,000,000) |  | 1,041,000,000 | 1,036,000,000 |
Liability Insurance for Damages |  |  |  | 800,000,000 |  |  | 800,000,000 |
Proceeds From Insurance Settlement | $ 297,000,000.0 | $ 108,000,000.0 |  |  |  |  | 800,000,000 |
Recorded reserves to cover environmental remediation at various sites |  |  |  | $ 92,600,000 |  | 104,400,000 | $ 92,600,000 |
Coal-fired Generating Capacity | MW |  |  |  | 2,080 |  |  | 2,080 |
Coal-fired Generating Capacity, Percent of Total Capacity | Rate |  |  |  | 72.00% |  |  |  |
Coal-fired Generating Capacity, Percent of Total Coal-Fired Capacity | Rate |  |  |  | 100.00% |  |  |  |
Wind Power Purchase Agreement, Purchase Percentage | Rate |  |  |  | 100.00% |  |  | 100.00% |
Nameplate Capacity | MW |  |  |  | 102 |  |  | 102 |
Expenses Other Than Third Party Claims |  |  |  | $ 22,000,000 |  | 420,000,000 | $ 442,000,000 |
Costs Resulting from the Greater Lawrence Incident |  |  | $ 1,023,000,000 | 17,000,000 |  | 1,461,000,000 | 1,478,000,000 |
Guaranty Liabilities |  |  |  | 40,700,000 |  |  | 40,700,000 |
Liability Associated with Schahfer Generating Station Retirement |  |  |  | 4,600,000 |  |  | 4,600,000 |
Restructuring and Related Cost, Expected Cost |  |  |  | $ 38,000,000 |  |  | $ 38,000,000 |
MGP Sites |  |  |  |  |  |  |  |
Other Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |
Number of waste disposal sites identified by program |  |  |  | 54 |  |  | 54 |
Liability for estimated remediation costs |  |  |  | $ 85,000,000.0 |  | 102,200,000 | $ 85,000,000.0 |
Reasonably possible remediation costs variance from reserve |  |  |  | $ 20,000,000 |  |  | 20,000,000 |
Columbia Of Massachusetts |  |  |  |  |  |  |  |
Other Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |
Gas Meters Affected |  |  |  | 7,500 |  |  |  |
Businesses Impacted by Incident |  |  |  | 700 |  |  |  |
Pipeline Replacement Expenses |  |  |  |  |  |  | 258,000,000 |
Deferred Tax Assets, Net |  |  |  | $ 50,000,000 |  |  | 50,000,000 |
Plea Agreement Criminal Fine |  |  |  | $ 53,030,116 |  |  |  |
Maximum | Columbia Of Massachusetts |  |  |  |  |  |  |  |
Other Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |
Expenses Related to Third-Party Claims, Fines, Penalties, and settlements |  |  |  |  |  |  | 1,050,000,000 |
Expenses Other Than Third Party Claims |  |  |  |  |  |  | 450,000,000 |
Minimum | Columbia Of Massachusetts |  |  |  |  |  |  |  |
Other Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |
Expenses Related to Third-Party Claims, Fines, Penalties, and settlements |  |  |  |  |  |  | 1,036,000,000 |
Expenses Other Than Third Party Claims |  |  |  |  |  |  | 445,000,000 |
Coal Transportation | Maximum |  |  |  |  |  |  |  |
Other Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |
Long Term Purchase Commitment Expiration Year |  |  |  | 2021 |  |  |  |
Pipeline Service Agreements [Member] | Maximum |  |  |  |  |  |  |  |
Other Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |
Long Term Purchase Commitment Expiration Year |  |  |  | 2038 |  |  |  |
Pipeline Service Agreements [Member] | Minimum |  |  |  |  |  |  |  |
Other Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |
Long Term Purchase Commitment Expiration Year |  |  |  | 2021 |  |  |  |
IT Service Agreements [Member] | Maximum |  |  |  |  |  |  |  |
Other Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |
Long Term Purchase Commitment Expiration Year |  |  |  | 2025 |  |  |  |
Standby Letters of Credit |  |  |  |  |  |  |  |
Other Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |
Line of Credit Facility, Amount Outstanding |  |  |  | $ 15,200,000 |  | $ 10,200,000 | $ 15,200,000 |

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