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Environmental Matters (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Air Quality [Abstract]    
Cost of most of the allowances granted to company for NOx and SO2 inventory usage $ 0  
SIGECO investment in Property, Plant and Equipment, Pollution control equipment 411.0  
Property, Plant and Equipment, amount of investment in pollution control equipment included in rate base 411.0  
Percentage of coal fired generating fleet currently being scrubbed for SO2 (in hundredths) 100.00%  
Percentage of coal fired generating fleet currently controlled for NOx (in hundredths) 90.00%  
Percentage reduction of SO2 per Cross-State Air Pollution Rule (in hundredths) 71.00%  
Percentage reduction of NOx per Cross-State Air Pollution Rule (in hundredths) 52.00%  
Climate Changes [Abstract]    
Maximum level of greenhouse gas emissions that prompts requirement to obtain permit for facilities to construct new facility of significant modification to existing facility (in tons) 75,000  
Coal Ash Waste Disposal and Ash Ponds [Abstract]    
Estimated capital expenditures to comply with ash pond and coal ash disposal regulations 30.0  
Potential estimated capital expenditures to comply with ash pond and coal ash disposal regulations with stringent alternative 100.0  
Estimated annual compliance costs maximum with ash pond and coal ash disposal regulation 5.0  
Clean Water Act [Abstract]    
Estimated capital expenditures related to Clean Water Act 40.0  
Jacobsville Superfund Site [Abstract]    
EPA identified sources of historic lead
 The EPA has identified four sources of historic lead contamination.  These four sources shut down manufacturing operations years ago.  When drawing up the boundaries for the listing, the EPA included a 250 acre block of properties surrounding the Jacobsville neighborhood, including the Company's operations center.  
  
Manufactured Gas Plants    
Site contingency, accrual, undiscounted amount 41.6  
Remediation cost accrued and not yet spent a the end of the period 6.5 5.5
Indiana Gas [Member]
    
Manufactured Gas Plants    
Number of sites identified with potential remedial responsibility for entity (in number of sites) 26  
Site contingency, accrual, undiscounted amount 23.1  
Environmental cost recognized, recover from insurance carriers credited to expense 20.8  
SIGECO [Member]
    
Manufactured Gas Plants    
Number of sites identified with potential remedial responsibility for entity (in number of sites) 5  
Site contingency, accrual, undiscounted amount 18.5  
Environmental cost recognized, recover from insurance carriers credited to expense $ 15.1