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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