Derivative Instruments and Hedging Activities (Fair Value and Balance Sheet Location (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Derivative Liability, Fair Value	$ 29.2
Derivative Liability, Fair Value, Amount Offset Against Collateral	16.8
THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative Liability, Fair Value	29.2
Derivative Liability, Fair Value, Amount Offset Against Collateral	16.8
Total Assets [Member]
Derivative Asset, Fair Value	20.8	$ 30.7
Derivative, Collateral, Obligation to Return Securities	(16.8)	(17.9)
Derivative, Collateral, Obligation to Return Cash	0.0	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	4.0	12.8
Total Assets [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative Asset, Fair Value	20.8	30.8
Derivative, Collateral, Obligation to Return Securities	(16.8)	(17.9)
Derivative, Collateral, Obligation to Return Cash	0.0	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	4.0	12.9
Total Liabilities [Member]
Derivative Liability, Fair Value	29.2	27.5
Derivative, Collateral, Right to Reclaim Securities	(16.8)	(17.9)
Derivative, Collateral, Right to Reclaim Cash	(6.3)	(8.0)
Derivative Liability, Fair Value, Amount Offset Against Collateral	6.1	1.6
Total Liabilities [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative Liability, Fair Value	29.2	27.5
Derivative, Collateral, Right to Reclaim Securities	(16.8)	(17.9)
Derivative, Collateral, Right to Reclaim Cash	(6.3)	(8.0)
Derivative Liability, Fair Value, Amount Offset Against Collateral	6.1	1.6
Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Prepayments and Current Assets [Member]
Derivative Asset, Fair Value	11.0	16.2
Derivative, Collateral, Obligation to Return Securities	(10.5)	(7.1)
Derivative, Collateral, Obligation to Return Cash	0.0	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	0.5	9.1
Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Prepayments and Current Assets [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative Asset, Fair Value	11.0	16.2
Derivative, Collateral, Obligation to Return Securities	(10.5)	(7.1)
Derivative, Collateral, Obligation to Return Cash	0.0	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	0.5	9.1
Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member]
Derivative Liability, Fair Value	16.4	7.1
Derivative, Collateral, Right to Reclaim Securities	(10.5)	(7.1)
Derivative, Collateral, Right to Reclaim Cash	(5.5)	0.0
Derivative Liability, Fair Value, Amount Offset Against Collateral	0.4	0.0
Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative Liability, Fair Value	16.4	7.1
Derivative, Collateral, Right to Reclaim Securities	(10.5)	(7.1)
Derivative, Collateral, Right to Reclaim Cash	(5.5)	0.0
Derivative Liability, Fair Value, Amount Offset Against Collateral	0.4	0.0
Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Asset [Member]
Derivative Asset, Fair Value	0.6	3.0
Derivative, Collateral, Obligation to Return Securities	(0.6)	(2.4)
Derivative, Collateral, Obligation to Return Cash	0.0	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	0.0	0.6
Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Asset [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative Asset, Fair Value	0.6	3.0
Derivative, Collateral, Obligation to Return Securities	(0.6)	(2.4)
Derivative, Collateral, Obligation to Return Cash	0.0	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	0.0	0.6
Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Credit [Member]
Derivative Liability, Fair Value	2.4	2.7
Derivative, Collateral, Right to Reclaim Securities	(0.6)	(2.4)
Derivative, Collateral, Right to Reclaim Cash	(0.8)	0.0
Derivative Liability, Fair Value, Amount Offset Against Collateral	1.0	0.3
Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Credit [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative Liability, Fair Value	2.4	2.7
Derivative, Collateral, Right to Reclaim Securities	(0.6)	(2.4)
Derivative, Collateral, Right to Reclaim Cash	(0.8)	0.0
Derivative Liability, Fair Value, Amount Offset Against Collateral	1.0	0.3
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] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member] | THE DAYTON POWER AND LIGHT COMPANY [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	1.2
Derivative, Collateral, Obligation to Return Securities	0.0
Derivative, Collateral, Obligation to Return Cash	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	1.2
Designated as Hedging Instrument [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	1.2
Derivative, Collateral, Obligation to Return Securities	0.0
Derivative, Collateral, Obligation to Return Cash	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	1.2
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	6.0	7.3
Derivative, Collateral, Obligation to Return Securities	(4.7)	(5.5)
Derivative, Collateral, Obligation to Return Cash	0.0	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	1.3	1.8
Not Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Prepayments and Current Assets [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative Asset, Fair Value	6.0	7.4
Derivative, Collateral, Obligation to Return Securities	(4.7)	(5.5)
Derivative, Collateral, Obligation to Return Cash	0.0	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	1.3	1.9
Not Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member]
Derivative Liability, Fair Value	7.7	14.5
Derivative, Collateral, Right to Reclaim Securities	(4.7)	(5.5)
Derivative, Collateral, Right to Reclaim Cash	0.0	(8.0)
Derivative Liability, Fair Value, Amount Offset Against Collateral	3.0	1.0
Not Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative Liability, Fair Value	7.7	14.5
Derivative, Collateral, Right to Reclaim Securities	(4.7)	(5.5)
Derivative, Collateral, Right to Reclaim Cash	0.0	(8.0)
Derivative Liability, Fair Value, Amount Offset Against Collateral	3.0	1.0
Not Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Asset [Member]
Derivative Asset, Fair Value	1.9	4.0
Derivative, Collateral, Obligation to Return Securities	(1.0)	(2.7)
Derivative, Collateral, Obligation to Return Cash	0.0	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	0.9	1.3
Not Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Asset [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative Asset, Fair Value	1.9	4.0
Derivative, Collateral, Obligation to Return Securities	(1.0)	(2.7)
Derivative, Collateral, Obligation to Return Cash	0.0	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	0.9	1.3
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	2.7
Derivative, Collateral, Right to Reclaim Securities	(1.0)	(2.7)
Derivative, Collateral, Right to Reclaim Cash	0.0	0.0
Derivative Liability, Fair Value, Amount Offset Against Collateral	1.0	0.0
Not Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Credit [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative Liability, Fair Value	2.0	2.7
Derivative, Collateral, Right to Reclaim Securities	(1.0)	(2.7)
Derivative, Collateral, Right to Reclaim Cash	0.0	0.0
Derivative Liability, Fair Value, Amount Offset Against Collateral	1.0	0.0
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	0.1	0.2
Derivative, Collateral, Obligation to Return Securities	0.0	(0.2)
Derivative, Collateral, Obligation to Return Cash	0.0	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	0.1	0.0
Not Designated as Hedging Instrument [Member] | Commodity Contract - FTR [Member] | Short-term Derivative Positions [Member] | Other Prepayments and Current Assets [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative Asset, Fair Value	0.1	0.2
Derivative, Collateral, Obligation to Return Securities	0.0	(0.2)
Derivative, Collateral, Obligation to Return Cash	0.0	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	0.1	0.0
Not Designated as Hedging Instrument [Member] | Commodity Contract - FTR [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member]
Derivative Liability, Fair Value	0.0	0.5
Derivative, Collateral, Right to Reclaim Securities	0.0	(0.2)
Derivative, Collateral, Right to Reclaim Cash	0.0	0.0
Derivative Liability, Fair Value, Amount Offset Against Collateral	0.0	0.3
Not Designated as Hedging Instrument [Member] | Commodity Contract - FTR [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative Liability, Fair Value	0.0	0.5
Derivative, Collateral, Right to Reclaim Securities	0.0	(0.2)
Derivative, Collateral, Right to Reclaim Cash	0.0	0.0
Derivative Liability, Fair Value, Amount Offset Against Collateral	$ 0.0	$ 0.3