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Regulatory Matters (Narrative) (Details) - USD ($)
$ in Millions
Apr. 26, 2017
Apr. 14, 2017
Mar. 17, 2017
Oct. 31, 2016
Jun. 30, 2016
Apr. 29, 2016
Dec. 31, 2016
Dec. 28, 2016
NIPSCO                
Regulatory Matters [Line Items]                
Public Utilities, Requested Rate Increase (Decrease), Amount         $ 45.5      
NIPSCO | TDSIC 5 Gas                
Regulatory Matters [Line Items]                
Regulatory Net Capital Expenditures Included In Filing         $ 211.6      
Seven Year Plan of Eligible Investments Under TDSIC               $ 824.0
NIPSCO | TDSIC 6 Gas                
Regulatory Matters [Line Items]                
Regulatory Net Capital Expenditures Included In Filing             $ 271.3  
NIPSCO | ECR 29                
Regulatory Matters [Line Items]                
Regulatory Net Capital Expenditures Included In Filing             $ 261.1  
Columbia Of Massachusetts | 2017 GSEP                
Regulatory Matters [Line Items]                
Public Utilities, Requested Rate Increase (Decrease), Cumulative Amount       $ 17.2        
Public Utilities, Requested Rate Increase (Decrease), Amount       $ 8.4        
Columbia Of Virginia                
Regulatory Matters [Line Items]                
Public Utilities, Requested Rate Increase (Decrease), Amount           $ 37.0    
Public Utilities, Approved Rate Increase (Decrease), Amount     $ 28.5          
Subsequent Event | Columbia Of Ohio                
Regulatory Matters [Line Items]                
Public Utilities, Approved Rate Increase (Decrease), Amount $ 31.5              
Subsequent Event | Columbia Of Maryland                
Regulatory Matters [Line Items]                
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount   $ 6.0