Derivative Instruments and Hedging Activities (Fair Value and Balance Sheet Location (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Total Assets [Member]
Derivative Asset, Fair Value	$ 1.9	$ 12.8
Derivative, Collateral, Obligation to Return Securities	0.0	(8.7)
Derivative, Collateral, Obligation to Return Cash	0.0	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	1.9	4.1
Total Liabilities [Member]
Derivative Liability, Fair Value	0.2	15.3
Derivative, Collateral, Right to Reclaim Securities	0.0	(8.7)
Derivative, Collateral, Right to Reclaim Cash	(0.1)	(1.4)
Liability position offset by the asset position of counterparties with master netting agreements	0.1	5.2
Cash Flow Hedging [Member] | Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Prepayments and Current Assets [Member]
Derivative Asset, Fair Value		4.9
Derivative, Collateral, Obligation to Return Securities		(4.9)
Derivative, Collateral, Obligation to Return Cash		0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral		0.0
Cash Flow Hedging [Member] | Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member]
Derivative Liability, Fair Value		9.0
Derivative, Collateral, Right to Reclaim Securities		(4.9)
Derivative, Collateral, Right to Reclaim Cash		(1.4)
Liability position offset by the asset position of counterparties with master netting agreements		2.7
Cash Flow Hedging [Member] | Interest Rate Swap [Member] | Short-term Derivative Positions [Member] | Other Prepayments and Current Assets [Member]
Derivative Asset, Fair Value	0.5
Derivative, Collateral, Obligation to Return Securities	0.0
Derivative, Collateral, Obligation to Return Cash	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	0.5
Cash Flow Hedging [Member] | Interest Rate Swap [Member] | Long-term Derivative Positions [Member] | Other Deferred Asset [Member]
Derivative Asset, Fair Value	1.4	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	1.4	1.8
Fair Value Hedging [Member] | Commodity Contract - FTR [Member] | Short-term Derivative Positions [Member] | Other Prepayments and Current Assets [Member]
Derivative Asset, Fair Value		0.2
Derivative, Collateral, Obligation to Return Securities		(0.1)
Derivative, Collateral, Obligation to Return Cash		0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral		0.1
Fair Value Hedging [Member] | Commodity Contract - FTR [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member]
Derivative Liability, Fair Value	0.1	0.3
Derivative, Collateral, Right to Reclaim Securities	0.0	0.0
Derivative, Collateral, Right to Reclaim Cash	0.0	0.0
Liability position offset by the asset position of counterparties with master netting agreements	0.1	0.3
Fair Value Hedging [Member] | Natural Gas [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member]
Derivative Liability, Fair Value		0.1
Derivative, Collateral, Right to Reclaim Securities		(0.1)
Derivative, Collateral, Right to Reclaim Cash		0.0
Liability position offset by the asset position of counterparties with master netting agreements		0.0
Fair Value Hedging [Member] | Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Prepayments and Current Assets [Member]
Derivative Asset, Fair Value		5.3
Derivative, Collateral, Obligation to Return Securities		(3.7)
Derivative, Collateral, Obligation to Return Cash		0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral		1.6
Fair Value Hedging [Member] | Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member]
Derivative Liability, Fair Value	0.1	5.9
Derivative, Collateral, Right to Reclaim Securities	0.0	(3.7)
Derivative, Collateral, Right to Reclaim Cash	(0.1)	0.0
Liability position offset by the asset position of counterparties with master netting agreements	0.0	2.2
Fair Value Hedging [Member] | Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Asset [Member]
Derivative Asset, Fair Value		0.6
Derivative, Collateral, Obligation to Return Securities		0.0
Derivative, Collateral, Obligation to Return Cash		0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral		0.6
Designated as Hedging Instrument [Member] | Interest Rate Swap [Member] | Other Deferred Asset [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative Asset, Fair Value	$ 1.9	$ 1.8