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Regulatory Matters (Narrative) (Details) - USD ($)
$ in Millions
 12 Months Ended 156 Months Ended
Oct. 31, 2018
Oct. 25, 2018
Oct. 09, 2018
Aug. 28, 2018
Apr. 18, 2018
Apr. 13, 2018
Feb. 13, 2018
Nov. 08, 2017
Dec. 27, 2016
Dec. 31, 2018
Dec. 31, 2017
Regulatory Matters [Line Items]                      
Public Utilities, Requested Rate Increase (Decrease), Adjustment       $ 5.2              
Remaining Recovery Period of Regulatory Assets for which No Return on Investment During Recovery Period is Provided                   41 years  
Regulatory asset not earning return on investment                   $ 1,552.6  
Expenses recovered as components of cost of service and regulatory orders                   1,917.1  
Regulatory assets Requiring Specific Rate Action                   320.4  
Public Utilities, Requested Rate Increase (Decrease), Waiver Amount in excess of Cap $ 2.9                    
Columbia Of Ohio                      
Regulatory Matters [Line Items]                      
Public Utilities, Requested Rate Increase (Decrease), Amount   $ 74.5     $ 70.0            
Public Utilities, Requested Biennial Rate Increase (Decrease), Amount increases         $ 98.0            
Depreciation Before Rate Regulation                     $ 806.8
Decrease In Depreciation And Amortization Over That Reflected In Rates                     92.2
Depreciation Regulatory Asset                   39.5 49.3
Requested IRP Extension                 5 years    
Columbia Of Virginia                      
Regulatory Matters [Line Items]                      
Public Utilities, Requested Rate Increase (Decrease), Amount [1]       $ 14.2              
NIPSCO                      
Regulatory Matters [Line Items]                      
Public Utilities, Approved Rate Increase (Decrease), Amount               $ 91.5      
Regulatory Net Capital Expenditures Included In Filing               $ 35.5      
Public Utilities, Requested Incremental Capital Investment                   228.8  
Columbia Of Massachusetts                      
Regulatory Matters [Line Items]                      
Public Utilities, Requested Rate Increase (Decrease), Amount           $ 24.1          
Public Utilities, Customer Refund for Tax Rate Change Due to TCJA     $ 95.8                
Columbia Of Maryland                      
Regulatory Matters [Line Items]                      
Public Utilities, Requested Rate Increase (Decrease), Amount           4.6          
Public Utilities, Approved Rate Increase (Decrease), Amount           $ 2.2 $ (1.3)        
IRP | Columbia Of Ohio                      
Regulatory Matters [Line Items]                      
Deferred Depreciation                   29.1 26.5
Capital Expenditure Program | Columbia Of Ohio                      
Regulatory Matters [Line Items]                      
Deferred Depreciation                   76.0 49.8
EERM [Domain] | NIPSCO                      
Regulatory Matters [Line Items]                      
Deferred Depreciation                   14.4 13.9
TDSIC [Domain] | NIPSCO                      
Regulatory Matters [Line Items]                      
Deferred Depreciation                   16.5 10.3
Deferred Post-in-Service Carrying Charges                   9.5 8.7
IRP and CEP [Domain] | Columbia Of Ohio                      
Regulatory Matters [Line Items]                      
Deferred Post-in-Service Carrying Charges                   $ 197.1 $ 164.6
[1] Rates implemented subject to refund pending a final order from the VSCC.