Derivative Instruments and Hedging Activities (Fair Value and Balance Sheet Location (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Total Assets [Member]
Derivative Asset, Fair Value	$ 2.2	$ 1.8
Derivative, Collateral, Obligation to Return Securities	0.0	0.0
Derivative, Collateral, Obligation to Return Cash	0.0	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	2.2	1.8
Total Assets [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative Asset, Fair Value	2.2	1.8
Derivative, Collateral, Obligation to Return Securities	0.0	0.0
Derivative, Collateral, Obligation to Return Cash	0.0	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	2.2	1.8
Cash Flow Hedging [Member] \| Interest Rate Swap [Member] \| Short-term Derivative Positions [Member] \| Other Prepayments and Current Assets [Member]
Derivative Asset, Fair Value	0.9
Derivative, Collateral, Obligation to Return Securities	0.0
Derivative, Collateral, Obligation to Return Cash	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	0.9
Cash Flow Hedging [Member] \| Interest Rate Swap [Member] \| Short-term Derivative Positions [Member] \| Other Prepayments and Current Assets [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative Asset, Fair Value	0.9
Derivative, Collateral, Obligation to Return Securities	0.0
Derivative, Collateral, Obligation to Return Cash	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	0.9
Cash Flow Hedging [Member] \| Interest Rate Swap [Member] \| Long-term Derivative Positions [Member] \| Other Deferred Asset [Member]
Derivative Asset, Fair Value	1.3	1.8
Derivative, Collateral, Obligation to Return Securities	0.0	0.0
Derivative, Collateral, Obligation to Return Cash	0.0	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	1.3	1.8
Cash Flow Hedging [Member] \| Interest Rate Swap [Member] \| Long-term Derivative Positions [Member] \| Other Deferred Asset [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative Asset, Fair Value	1.3	1.8
Derivative, Collateral, Obligation to Return Securities		0.0
Derivative, Collateral, Obligation to Return Cash		0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	$ 1.3	$ 1.8

Amount of obligation to return cash collateral under master netting arrangements that have not been offset against derivative assets.

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