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Other Commitments And Contingencies (Narrative) (Details)
 3 Months Ended 4 Months Ended 31 Months Ended
Mar. 31, 2021
USD ($)
Rate
Dec. 31, 2018
USD ($)
Mar. 31, 2021
USD ($)
Rate
Dec. 31, 2020
USD ($)
Jul. 26, 2019
USD ($)
Other Commitments And Contingencies [Line Items]          
Guaranty Liabilities $ 34,000,000.0   $ 34,000,000.0    
Litigation Settlement, Amount Awarded to Other Party         $ 143,000,000
Recorded reserves to cover environmental remediation at various sites $ 90,600,000   $ 90,600,000 $ 92,600,000  
Wind Power Purchase Agreement, Purchase Percentage | Rate 100.00%   100.00%    
Costs Resulting from the Greater Lawrence Incident   $ 1,023,000,000      
Restructuring and Related Cost, Expected Cost $ 40,000,000   $ 40,000,000    
Restructuring and Related Cost, Cost Incurred to Date $ 38,100,000   $ 38,100,000    
MGP Sites          
Other Commitments And Contingencies [Line Items]          
Number of waste disposal sites identified by program 54   54    
Liability for Estimated Remediation Costs $ 84,300,000   $ 84,300,000 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        
Plea Agreement Criminal Fine $ 53,030,116        
Pipeline Replacement Expenses     258,000,000    
Deferred Tax Assets, Net 50,000,000   50,000,000    
Minimum | Columbia Of Massachusetts          
Other Commitments And Contingencies [Line Items]          
Expenses Related to Third-Party Claims     1,040,000,000    
Standby Letters of Credit          
Other Commitments And Contingencies [Line Items]          
Long-term Line of Credit $ 15,200,000   $ 15,200,000 $ 15,200,000