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Derivative Instruments and Hedging Activities (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2011
Sep. 30, 2012
Mar. 01, 2011
Gain (Loss) on Settlement of Derivative Instrument (Before Tax) $ 48.1    
Gain (Loss) on Settlement of Derivative Instrument (After Tax) 31.3    
Loans Payable to Bank, Noncurrent 425.0    
Debt instrument interest percentage     4.20%
Reclassified from accumulated OCI into income, forecasted transactions 5.1    
Reclassified from accumulated OCI into income, forecasted transactions, net of tax 3.3    
Fair value of commodity derivative instruments   22.2  
Liability position offset by the asset position of counterparties with master netting agreements   4.4  
Collateral Already Posted, Aggregate Fair Value   12.6  
Collateral if debt were to fall below investment grade   5.2  
DP&L [Member]
     
Fair value of commodity derivative instruments   19.8  
Liability position offset by the asset position of counterparties with master netting agreements   4.4  
Collateral Already Posted, Aggregate Fair Value   10.2  
Collateral if debt were to fall below investment grade   5.2  
Planned Future Borrowing Activities In 2011 [Member]
     
Interest rate hedging relationships notional amount 200.0    
Senior Notes Maturing in September 2011 - 6.875% [Member]
     
Senior notes maturing in September 2011 $ 297.4    
Debt instrument interest percentage 6.875%