Derivative Instruments and Hedging Activities (Gains or Losses Recognized in AOCI for the Cash Flow Hedges) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Forward Contract Power [Member]
Beginning accumulated derivative gain / (loss) in AOCI	$ 1.4	$ (3.0)	$ 0.3
Net gains / (losses) associated with current period hedging transactions	(19.0)	1.0	(2.6)
Ending accumulated derivative gain / (loss) in AOCI	0.2	1.4	(3.0)
Portion expected to be reclassified to earnings in the next twelve months	3.5
Maximum length of time that we are hedging our exposure to variability in future cash flows related to forecasted transactions (in months)	24 months
Interest Rate Contract [Member]
Beginning accumulated derivative gain / (loss) in AOCI	19.2	0.5	(0.8)
Net gains / (losses) associated with current period hedging transactions		18.7	1.1
Ending accumulated derivative gain / (loss) in AOCI	18.3	19.2	0.5
Net gains / (losses) associated with the ineffective portion of the hedging transaction		0.8	0.2
Portion expected to be reclassified to earnings in the next twelve months	(0.9)
Maximum length of time that we are hedging our exposure to variability in future cash flows related to forecasted transactions (in months)	0 months
DP&L [Member] \| Forward Contract Power [Member]
Beginning accumulated derivative gain / (loss) in AOCI	1.0	(4.7)	(0.8)
Net gains / (losses) associated with current period hedging transactions	(18.8)	1.0	(3.0)
Ending accumulated derivative gain / (loss) in AOCI	0.2	1.0	(4.7)
Portion expected to be reclassified to earnings in the next twelve months	3.5
Maximum length of time that we are hedging our exposure to variability in future cash flows related to forecasted transactions (in months)	24 months
DP&L [Member] \| Interest Rate Contract [Member]
Beginning accumulated derivative gain / (loss) in AOCI	5.2			9.8
Ending accumulated derivative gain / (loss) in AOCI	2.6		7.3	9.8
Portion expected to be reclassified to earnings in the next twelve months	(2.6)
Maximum length of time that we are hedging our exposure to variability in future cash flows related to forecasted transactions (in months)	0 months
Interest Expense [Member] \| Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	(0.9)		0.2
Interest Expense [Member] \| DP&L [Member] \| Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	(2.6)	(2.1)	(2.5)
Revenue [Member] \| Forward Contract Power [Member]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	18.3	2.1	(0.7)
Revenue [Member] \| DP&L [Member] \| Forward Contract Power [Member]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	18.2	1.4	(1.1)
Purchased Power [Member] \| Forward Contract Power [Member]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	(0.5)	1.3
Purchased Power [Member] \| DP&L [Member] \| Forward Contract Power [Member]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	$ (0.2)	$ 3.3	$ 0.2

Accumulated change, net of tax, in accumulated gains and losses from derivative instruments designated and qualifying as the effective portion of cash flow hedges. Includes an entity's share of an equity investee's Increase or Decrease in deferred hedging gains or losses.

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The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying as hedging instruments.

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The portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments representing (a) the amount of the hedge ineffectiveness and (b) the amount, if any, excluded from the assessment of hedge effectiveness.

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The effective portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments that was recognized in other comprehensive income during the current period.

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The estimated net amount of unrealized gains or losses on interest rate cash flow hedges as of the balance sheet date expected to be reclassified to earnings within the next twelve months.

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Maximum length of time over which the entity is hedging its exposure to the variability in future cash flows for forecasted transactions, excluding those forecasted transactions related to the payment of variable interest on existing financial instruments, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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