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Other Commitments And Contingencies (Narrative) (Details)
 3 Months Ended 4 Months Ended 16 Months Ended 19 Months Ended
Oct. 31, 2019
MW
Mar. 31, 2020
USD ($)
Rate
MW
Dec. 31, 2018
USD ($)
Dec. 31, 2019
USD ($)
Mar. 31, 2020
USD ($)
MW
Jul. 26, 2019
USD ($)
Jan. 01, 2019
Rate
MW
Other Commitments And Contingencies [Line Items]              
Guaranty Liabilities   $ 8,500,000     $ 8,500,000    
Gas Meters moved from inside to outside 2,200            
Litigation Settlement, Amount Awarded to Other Party           $ 143,000,000  
Expenses Related to Third-Party Claims   0   $ 1,041,000,000 1,041,000,000    
Liability Insurance for Damages   800,000,000     800,000,000    
Proceeds From Insurance Settlement         800,000,000    
Recorded reserves to cover environmental remediation at various sites   $ 91,400,000   104,400,000 $ 91,400,000    
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%          
Wind Power Purchase Agreement, Purchase Percentage | Rate             100.00%
Planned Replacement Capacity | MW   1,600     1,600    
Other incident-related costs   $ 9,000,000   420,000,000 $ 429,000,000    
Costs Resulting from the Greater Lawrence Incident   $ 9,000,000 $ 1,023,000,000 1,461,000,000 $ 1,470,000,000    
MGP Sites              
Other Commitments And Contingencies [Line Items]              
Number of waste disposal sites identified by program   63     63    
Liability for Estimated Remediation Costs   $ 86,800,000   102,200,000 $ 86,800,000    
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          
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    
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         1,055,000,000    
Other incident-related costs         460,000,000    
Minimum | Columbia Of Massachusetts              
Other Commitments And Contingencies [Line Items]              
Expenses Related to Third-Party Claims         1,041,000,000    
Other incident-related costs         450,000,000    
Standby Letters of Credit              
Other Commitments And Contingencies [Line Items]              
Long-term Line of Credit   $ 10,200,000   $ 10,200,000 $ 10,200,000    
Build Transfer Agreement [Member]              
Other Commitments And Contingencies [Line Items]              
Nameplate Capacity | MW 300           100
Purchase Power Agreements - Jan 2019 [Member]              
Other Commitments And Contingencies [Line Items]              
Long-term Purchase Commitment, Period   20 years