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Other Commitments And Contingencies (Narrative) (Details)
$ in Millions
 3 Months Ended
Mar. 31, 2017
USD ($)
Mar. 31, 2016
USD ($)
Dec. 31, 2016
USD ($)
Other Commitments And Contingencies [Line Items]      
Proposed Pipeline Safety Rule, time period for investigation and repair/replace activity on facilities 15 years    
Recorded reserves to cover environmental remediation at various sites $ 109.3   $ 111.4
Estimated Cost of Compliance with Coal Combustion Residuals, Maximum 230.0    
Estimated Cost of Compliance with Effluent Limitations Guidelines $ 170.0    
Transmission Upgrade Agreement Multiplier 1.71    
Liability Associated with Bailey Generating Station Retirement     22.1
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount $ 0.0    
Payments Pure Air 5.5 $ 4.9  
Liability Associated with Bailly Generating Station Retirement - Pure Air Contract Termination Portion $ 16.0    
MGP Sites      
Other Commitments And Contingencies [Line Items]      
Number of waste disposal sites identified by program 64    
Liability for Estimated Remediation Costs $ 103.9   105.5
Reasonably possible remediation costs variance from reserve $ 25.0    
NIPSCO      
Other Commitments And Contingencies [Line Items]      
Upgrade Sponsors Liability related to Portion Collected for Taxes on TUA     8.6
Maximum      
Other Commitments And Contingencies [Line Items]      
Ozone Level 75 ppb    
Minimum      
Other Commitments And Contingencies [Line Items]      
Ozone Level 70 ppb    
Minimum | NIPSCO      
Other Commitments And Contingencies [Line Items]      
Cost to Maintain and Operate Current Fleet of Coal-Fired Generating Units $ 1,000.0    
Standby Letters of Credit      
Other Commitments And Contingencies [Line Items]      
Long-term Line of Credit $ 14.7   $ 14.7