Derivative Instruments and Hedging Activities (Fair Value and Balance Sheet Location (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Derivative Liability, Fair Value	$ 15.3
Derivative Liability, Fair Value, Amount Offset Against Collateral	1.4
Total Assets [Member]
Derivative Asset, Fair Value	12.8	$ 20.8
Derivative, Collateral, Obligation to Return Securities	(8.7)	(16.8)
Derivative, Collateral, Obligation to Return Cash	0.0	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	4.1	4.0
Total Liabilities [Member]
Derivative Liability, Fair Value	15.3	29.2
Derivative, Collateral, Right to Reclaim Securities	(8.7)	(16.8)
Derivative, Collateral, Right to Reclaim Cash	(1.4)	(6.3)
Derivative Liability, Fair Value, Amount Offset Against Collateral	5.2	6.1
Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Prepayments and Current Assets [Member]
Derivative Asset, Fair Value	4.9	11.0
Derivative, Collateral, Obligation to Return Securities	(4.9)	(10.5)
Derivative, Collateral, Obligation to Return Cash	0.0	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	0.0	0.5
Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member]
Derivative Liability, Fair Value	9.0	16.4
Derivative, Collateral, Right to Reclaim Securities	(4.9)	(10.5)
Derivative, Collateral, Right to Reclaim Cash	(1.4)	(5.5)
Derivative Liability, Fair Value, Amount Offset Against Collateral	2.7	0.4
Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Asset [Member]
Derivative Asset, Fair Value	0.0	0.6
Derivative, Collateral, Obligation to Return Securities	0.0	(0.6)
Derivative, Collateral, Obligation to Return Cash	0.0	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	0.0	0.0
Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Credit [Member]
Derivative Liability, Fair Value		2.4
Derivative, Collateral, Right to Reclaim Securities		(0.6)
Derivative, Collateral, Right to Reclaim Cash		(0.8)
Derivative Liability, Fair Value, Amount Offset Against Collateral		1.0
Designated as Hedging Instrument [Member] | Interest Rate Swap [Member] | Other Deferred Asset [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative Asset, Fair Value	1.8	1.2
Designated as Hedging Instrument [Member] | Interest Rate Swap [Member] | Other Current Liabilities [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative Liability, Fair Value	0.0	0.7
Designated as Hedging Instrument [Member] | Interest Rate Swap [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member]
Derivative Liability, Fair Value		0.7
Derivative, Collateral, Right to Reclaim Securities		0.0
Derivative, Collateral, Right to Reclaim Cash		0.0
Derivative Liability, Fair Value, Amount Offset Against Collateral		0.7
Designated as Hedging Instrument [Member] | Interest Rate Swap [Member] | Long-term Derivative Positions [Member] | Other Deferred Asset [Member]
Derivative Asset, Fair Value	1.8	1.2
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	1.8	1.2
Not Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Prepayments and Current Assets [Member]
Derivative Asset, Fair Value	5.3	6.0
Derivative, Collateral, Obligation to Return Securities	(3.7)	(4.7)
Derivative, Collateral, Obligation to Return Cash	0.0	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	1.6	1.3
Not Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member]
Derivative Liability, Fair Value	5.9	7.7
Derivative, Collateral, Right to Reclaim Securities	(3.7)	(4.7)
Derivative, Collateral, Right to Reclaim Cash	0.0	0.0
Derivative Liability, Fair Value, Amount Offset Against Collateral	2.2	3.0
Not Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Asset [Member]
Derivative Asset, Fair Value	0.6	1.9
Derivative, Collateral, Obligation to Return Securities	0.0	(1.0)
Derivative, Collateral, Obligation to Return Cash	0.0	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	0.6	0.9
Not Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Credit [Member]
Derivative Liability, Fair Value		2.0
Derivative, Collateral, Right to Reclaim Securities		(1.0)
Derivative, Collateral, Right to Reclaim Cash		0.0
Derivative Liability, Fair Value, Amount Offset Against Collateral		1.0
Not Designated as Hedging Instrument [Member] | Commodity Contract - FTR [Member] | Short-term Derivative Positions [Member] | Other Prepayments and Current Assets [Member]
Derivative Asset, Fair Value	0.2	0.1
Derivative, Collateral, Obligation to Return Securities	(0.1)	0.0
Derivative, Collateral, Obligation to Return Cash	0.0	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	0.1	$ 0.1
Not Designated as Hedging Instrument [Member] | Commodity Contract - FTR [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member]
Derivative Liability, Fair Value	0.3
Derivative, Collateral, Right to Reclaim Securities	0.0
Derivative, Collateral, Right to Reclaim Cash	0.0
Derivative Liability, Fair Value, Amount Offset Against Collateral	0.3
Not Designated as Hedging Instrument [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
Derivative Liability, Fair Value, Amount Offset Against Collateral	$ 0.0