Derivative Instruments and Hedging Activities (Narrative) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Derivative Liability, Fair Value, Gross Asset	$ 12.2
Liability position offset by the asset position of counterparties with master netting agreements	4.9
Collateral Already Posted, Aggregate Fair Value	6.6
Collateral if debt were to fall below investment grade	0.7
Derivative Asset, Current		5.4us-gaap_DerivativeAssetsCurrent
Derivative Asset, Noncurrent		8.0us-gaap_DerivativeAssetsNoncurrent
DP&L [Member]
Derivative Liability, Fair Value, Gross Asset	12.3
Liability position offset by the asset position of counterparties with master netting agreements	6.6
Collateral Already Posted, Aggregate Fair Value	4.9
Collateral if debt were to fall below investment grade	0.8
Short-term Derivative Positions [Member]
Amount No Longer Considered Derivative	0.1	0.9
Derivative Asset, Current	11.1	5.4
Long-term Derivative Positions [Member]
Amount No Longer Considered Derivative		0.1
Derivative Asset, Noncurrent		$ 8.0

Amount of derivative gain previously accounted for as MTM but now being accounted for as NPNS

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The aggregate fair value of additional assets that would be required to be posted as collateral for derivative instruments with credit-risk-related contingent features if the credit-risk-related contingent features were triggered at the end of the reporting period.

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The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features.

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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, expected to be settled within one year or normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, expected to be settled after one year or the normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Fair value of financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset offset against the right to receive collateral. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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Fair value of asset associated with financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.

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