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Other Commitments And Contingencies (Narrative) (Details)
 3 Months Ended 4 Months Ended 9 Months Ended 16 Months Ended 25 Months Ended
Sep. 30, 2020
USD ($)
Rate
MW
Dec. 31, 2018
USD ($)
Sep. 30, 2020
USD ($)
Rate
MW
Dec. 31, 2019
USD ($)
Sep. 30, 2020
USD ($)
Rate
MW
Oct. 16, 2020
MW
Oct. 09, 2020
Oct. 31, 2019
MW
Jul. 26, 2019
USD ($)
Jan. 01, 2019
Rate
MW
Other Commitments And Contingencies [Line Items]                    
Guaranty Liabilities $ 195,600,000   $ 195,600,000   $ 195,600,000          
Litigation Settlement, Amount Awarded to Other Party                 $ 143,000,000  
Expenses Related to Third-Party Claims (3,000,000)   (5,000,000) $ 1,041,000,000 1,036,000,000          
Liability Insurance for Damages 800,000,000   800,000,000   800,000,000          
Proceeds From Insurance Settlement         800,000,000          
Recorded reserves to cover environmental remediation at various sites 93,400,000   $ 93,400,000 104,400,000 93,400,000          
Coal-fired Generating Capacity | MW     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%              
Liability Associated with Schahfer Generating Station Retirement $ 4,600,000   $ 4,600,000   $ 4,600,000          
Wind Power Purchase Agreement, Purchase Percentage | Rate 100.00%   100.00%   100.00%         100.00%
Planned Replacement Capacity | MW 1,400   1,400   1,400          
Other incident-related costs $ 5,000,000   $ 21,000,000 420,000,000 $ 441,000,000          
Costs Resulting from the Greater Lawrence Incident 2,000,000 $ 1,023,000,000 16,000,000 1,461,000,000 1,477,000,000          
Restructuring and Related Cost, Expected Cost $ 38,000,000   38,000,000   $ 38,000,000          
Other Restructuring Costs     $ 23,400,000              
MGP Sites                    
Other Commitments And Contingencies [Line Items]                    
Number of waste disposal sites identified by program 63   63   63          
Liability for Estimated Remediation Costs $ 88,300,000   $ 88,300,000 102,200,000 $ 88,300,000          
Reasonably possible remediation costs variance from reserve 20,000,000   $ 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              
Settlement Agreement Payment in Lieu of Penalties     $ 56,000,000.0              
Plea Agreement Criminal Fine     $ 53,030,116              
Miles of Affected Cast Iron and Bare Steel Pipeline System     45              
Pipeline Replacement Expenses         258,000,000          
Deferred Tax Assets, Net 50,000,000   $ 50,000,000   50,000,000          
NIPSCO                    
Other Commitments And Contingencies [Line Items]                    
Estimated Cost of Compliance with Effluent Limitations Guidelines     170,000,000              
Maximum | Columbia Of Massachusetts                    
Other Commitments And Contingencies [Line Items]                    
Civil Penalty Assessed for Violation of Federal Pipeline Safety Regulations     218,647              
Civil Penalty Assessed for Series of Violations of Federal Pipeline Safety Regulations     2,200,000              
Penalty Per Violation of Emergency Response Plan     250,000              
Penalty for Violations of Emergency Response Plan     20,000,000.0              
Penalty Per Violation of Operational Directives During Restoration Efforts     1,000,000.0              
Expenses Related to Third-Party Claims         1,050,000,000          
Other incident-related costs         455,000,000          
Minimum | Columbia Of Massachusetts                    
Other Commitments And Contingencies [Line Items]                    
Expenses Related to Third-Party Claims         1,036,000,000          
Other incident-related costs         445,000,000          
Standby Letters of Credit                    
Other Commitments And Contingencies [Line Items]                    
Long-term Line of Credit $ 10,200,000   $ 10,200,000 $ 10,200,000 $ 10,200,000          
Subsequent Event | MGP Sites                    
Other Commitments And Contingencies [Line Items]                    
Number of waste disposal sites identified by program             54      
Purchase Power Agreements - Jun 2020 [Member]                    
Other Commitments And Contingencies [Line Items]                    
Long-term Purchase Commitment, Period     20 years              
Nameplate Capacity | MW 500   500   500          
Storage Capacity | MW 30   30   30          
Build Transfer Agreement [Member]                    
Other Commitments And Contingencies [Line Items]                    
Nameplate Capacity | MW               300   100
Build Transfer Agreement [Member] | Subsequent Event                    
Other Commitments And Contingencies [Line Items]                    
Nameplate Capacity | MW           900        
Storage Capacity | MW           135        
Build Transfer Agreement [Member] | Subsequent Event | 2023 [Member]                    
Other Commitments And Contingencies [Line Items]                    
Nameplate Capacity | MW           635        
Build Transfer Agreement [Member] | Subsequent Event | 2022 [Member]                    
Other Commitments And Contingencies [Line Items]                    
Nameplate Capacity | MW           265        
Purchase Power Agreements - Jan 2019 [Member]                    
Other Commitments And Contingencies [Line Items]                    
Long-term Purchase Commitment, Period     20 years              
Nameplate Capacity | MW                   400