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Derivative Instruments and Hedging Activities (Fair Values of Derivative Instruments Designated as Hedging Instruments) (Details) (USD $)
In Millions
Sep. 30, 2011
Dec. 31, 2010
Derivative Liability, Fair Value$ (13.1) 
DP&L [Member] | Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Designated as Hedging Instrument [Member]
  
Derivative Asset, Fair Value0.3[1] 
Derivative Fair Value Counterparty And Collateral Netting0.3[2]1.0[2]
Derivative Fair Value Counterparty And Collateral Netting, Liability0.2[2] 
Derivative Liability, Fair Value(1.1)[1](2.8)[1]
DP&L [Member] | Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Prepayments and Current Assets [Member]
  
Derivative Asset, Fair Value, Amount Not Offset Against Collateral  
DP&L [Member] | Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member]
  
Derivative Liability, Fair Value, Not Offset Amount(0.9)(1.8)
DP&L [Member] | Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Designated as Hedging Instrument [Member]
  
Derivative Asset, Fair Value1.4[1]0.2[1]
Derivative Fair Value Counterparty And Collateral Netting(1.0)[2](0.2)[2]
Derivative Fair Value Counterparty And Collateral Netting, Liability0.1[2]0.1[2]
Derivative Liability, Fair Value(0.1)[1](0.2)[1]
DP&L [Member] | Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Credit [Member]
  
Derivative Liability, Fair Value, Not Offset Amount (0.1)
DP&L [Member] | Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Asset [Member]
  
Derivative Asset, Fair Value, Amount Not Offset Against Collateral0.4 
Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Designated as Hedging Instrument [Member]
  
Derivative Asset, Fair Value0.3[1] 
Derivative Fair Value Counterparty And Collateral Netting(0.3)[2]1.0[2]
Derivative Fair Value Counterparty And Collateral Netting, Liability0.2[2] 
Derivative Liability, Fair Value(1.1)[1](2.8)[1]
Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Prepayments and Current Assets [Member]
  
Derivative Asset, Fair Value, Amount Not Offset Against Collateral  
Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member]
  
Derivative Liability, Fair Value, Not Offset Amount(0.9)(1.8)
Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Designated as Hedging Instrument [Member]
  
Derivative Asset, Fair Value1.4[1]0.2[1]
Derivative Fair Value Counterparty And Collateral Netting(1.0)[2](0.2)[2]
Derivative Fair Value Counterparty And Collateral Netting, Liability0.1[2]0.1[2]
Derivative Liability, Fair Value(0.2)[1](0.2)[1]
Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Credit [Member]
  
Derivative Liability, Fair Value, Not Offset Amount(0.1)(0.1)
Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Asset [Member]
  
Derivative Asset, Fair Value, Amount Not Offset Against Collateral0.4 
Interest Rate Swap [Member] | Short-term Derivative Positions [Member] | Designated as Hedging Instrument [Member]
  
Derivative Fair Value Counterparty And Collateral Netting [2] [2]
Derivative Liability, Fair Value(28.5)[1](6.6)[1]
Interest Rate Swap [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member]
  
Derivative Liability, Fair Value, Not Offset Amount(28.5)(6.6)
Interest Rate Swap [Member] | Long-term Derivative Positions [Member] | Designated as Hedging Instrument [Member]
  
Derivative Asset, Fair Value 20.7[1]
Derivative Fair Value Counterparty And Collateral Netting  [2]
Interest Rate Swap [Member] | Long-term Derivative Positions [Member] | Other Deferred Credit [Member]
  
Derivative Liability, Fair Value, Not Offset Amount (20.7)
DP&L [Member] | Cash Flow Hedge [Member] | Short-term Derivative Positions [Member] | Designated as Hedging Instrument [Member]
  
Derivative Fair Value Counterparty And Collateral Netting(0.1)[2] 
Derivative, Fair Value, Net(0.8)[1](2.8)[1]
Derivative, Collateral, net 1.0[2]
Derivative, Fair Value, Offset, Net(0.9)(1.8)
DP&L [Member] | Cash Flow Hedge [Member] | Long-term Derivative Positions [Member] | Designated as Hedging Instrument [Member]
  
Derivative Fair Value Counterparty And Collateral Netting(0.9)[2] 
Derivative, Fair Value, Net1.3[1] 
Derivative, Collateral, net (0.1)[2]
Derivative, Fair Value, Offset, Net0.4(0.1)
DP&L [Member] | Cash Flow Hedge [Member] | Designated as Hedging Instrument [Member]
  
Derivative Fair Value Counterparty And Collateral Netting(1.0)[2] 
Derivative, Fair Value, Net0.5[1](2.8)[1]
Derivative, Collateral, net 0.9[2]
Derivative, Fair Value, Offset, Net(0.5)(1.9)
Cash Flow Hedge [Member] | Short-term Derivative Positions [Member] | Designated as Hedging Instrument [Member]
  
Derivative Fair Value Counterparty And Collateral Netting 1.0[2]
Derivative Liability, Fair Value, Not Offset Amount (8.4)
Derivative, Fair Value, Net(0.8)[1](9.4)[1]
Derivative, Collateral, net(0.1)[2] 
Derivative, Fair Value, Offset, Net(0.9) 
Cash Flow Hedge [Member] | Long-term Derivative Positions [Member] | Designated as Hedging Instrument [Member]
  
Derivative Fair Value Counterparty And Collateral Netting (0.1)[2]
Derivative Asset, Fair Value, Amount Not Offset Against Collateral 20.6
Derivative, Fair Value, Net(27.3)[1]20.7[1]
Derivative, Collateral, net(0.9)[2] 
Derivative, Fair Value, Offset, Net(28.2) 
Cash Flow Hedge [Member] | Designated as Hedging Instrument [Member]
  
Derivative Fair Value Counterparty And Collateral Netting 0.9[2]
Derivative, Fair Value, Net(28.1)[1]11.3[1]
Derivative, Collateral, net(1.0)[2] 
Derivative, Fair Value, Offset, Net(29.1)12.2
DP&L [Member]
  
Derivative Liability, Fair Value(8.5) 
Derivative, Fair Value, Offset, Net$ 2.1 
[1] 1 Includes credit valuation adjustment.
[2] 2 Includes counterparty and collateral netting.