Derivative Instruments and Hedging Activities (Fair Value and Balance Sheet Location (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Derivative Liability, Fair Value	$ 25.1
Liability position offset by the asset position of counterparties with master netting agreements	18.6
THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative Liability, Fair Value	25.2
Liability position offset by the asset position of counterparties with master netting agreements	18.6
Total Assets [Member]
Derivative Asset, Fair Value	30.4	$ 14.9
Derivative, Collateral, Obligation to Return Securities	(18.6)	(6.6)
Derivative, Collateral, Obligation to Return Cash	0.0	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	11.8	8.3
Total Assets [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative Asset, Fair Value	30.6	15.1
Derivative, Collateral, Obligation to Return Securities	(18.6)	(6.6)
Derivative, Collateral, Obligation to Return Cash	0.0	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	12.0	8.5
Total Liabilities [Member]
Derivative Liability, Fair Value	25.1	12.2
Derivative, Collateral, Right to Reclaim Securities	(18.6)	(6.6)
Derivative, Collateral, Right to Reclaim Cash	(5.6)	(4.9)
Liability position offset by the asset position of counterparties with master netting agreements	0.9	0.7
Total Liabilities [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative Liability, Fair Value	25.2	12.3
Derivative, Collateral, Right to Reclaim Securities	(18.6)	(6.6)
Derivative, Collateral, Right to Reclaim Cash	(5.6)	(4.9)
Liability position offset by the asset position of counterparties with master netting agreements	1.0	0.8
Cash Flow Hedging [Member] | Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Prepayments and Current Assets [Member]
Derivative Asset, Fair Value	10.8	5.6
Derivative, Collateral, Obligation to Return Securities	(6.0)	(2.0)
Derivative, Collateral, Obligation to Return Cash	0.0	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	4.8	3.6
Cash Flow Hedging [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	10.8	5.6
Derivative, Collateral, Obligation to Return Securities	(6.0)	(2.0)
Derivative, Collateral, Obligation to Return Cash	0.0	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	4.8	3.6
Cash Flow Hedging [Member] | Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member]
Derivative Liability, Fair Value	6.0	2.1
Derivative, Collateral, Right to Reclaim Securities	(6.0)	(2.0)
Derivative, Collateral, Right to Reclaim Cash	0.0	0.0
Liability position offset by the asset position of counterparties with master netting agreements	0.0	0.1
Cash Flow Hedging [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	6.0	2.1
Derivative, Collateral, Right to Reclaim Securities	(6.0)	(2.0)
Derivative, Collateral, Right to Reclaim Cash	0.0	0.0
Liability position offset by the asset position of counterparties with master netting agreements	0.0	0.1
Cash Flow Hedging [Member] | Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Asset [Member]
Derivative Asset, Fair Value	8.2	0.3
Derivative, Collateral, Obligation to Return Securities	(3.0)	(0.3)
Derivative, Collateral, Obligation to Return Cash	0.0	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	5.2	0.0
Cash Flow Hedging [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	8.2	0.3
Derivative, Collateral, Obligation to Return Securities	(3.0)	(0.3)
Derivative, Collateral, Obligation to Return Cash	0.0	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	5.2	0.0
Cash Flow Hedging [Member] | Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Credit [Member]
Derivative Liability, Fair Value	3.0	0.6
Derivative, Collateral, Right to Reclaim Securities	(3.0)	(0.3)
Derivative, Collateral, Right to Reclaim Cash	0.0	(0.3)
Liability position offset by the asset position of counterparties with master netting agreements	0.0	0.0
Cash Flow Hedging [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	3.0	0.6
Derivative, Collateral, Right to Reclaim Securities	(3.0)	(0.3)
Derivative, Collateral, Right to Reclaim Cash	0.0	(0.3)
Liability position offset by the asset position of counterparties with master netting agreements	0.0	0.0
Fair Value Hedging [Member] | Commodity Contract - Heating Oil [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member]
Derivative Liability, Fair Value		0.4
Derivative, Collateral, Right to Reclaim Securities		0.0
Derivative, Collateral, Right to Reclaim Cash		(0.4)
Liability position offset by the asset position of counterparties with master netting agreements		0.0
Fair Value Hedging [Member] | Commodity Contract - Heating Oil [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative Liability, Fair Value		0.4
Derivative, Collateral, Right to Reclaim Securities		0.0
Derivative, Collateral, Right to Reclaim Cash		(0.4)
Liability position offset by the asset position of counterparties with master netting agreements		0.0
Fair Value Hedging [Member] | Commodity Contract - FTR [Member] | Short-term Derivative Positions [Member] | Other Prepayments and Current Assets [Member]
Derivative Asset, Fair Value	0.4
Derivative, Collateral, Obligation to Return Securities	(0.4)
Derivative, Collateral, Obligation to Return Cash	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	0.0
Fair Value Hedging [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	0.4
Derivative, Collateral, Obligation to Return Securities	(0.4)
Derivative, Collateral, Obligation to Return Cash	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	0.0
Fair Value Hedging [Member] | Commodity Contract - FTR [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member]
Derivative Liability, Fair Value	0.7	0.6
Derivative, Collateral, Right to Reclaim Securities	(0.4)	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	0.3	0.6
Fair Value Hedging [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	0.7	0.6
Derivative, Collateral, Right to Reclaim Securities	(0.4)	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	0.3	0.6
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	5.5
Derivative, Collateral, Obligation to Return Securities	(4.0)	(3.4)
Derivative, Collateral, Obligation to Return Cash	0.0	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	1.3	2.1
Fair Value Hedging [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	5.4	5.6
Derivative, Collateral, Obligation to Return Securities	(4.0)	(3.4)
Derivative, Collateral, Obligation to Return Cash	0.0	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	1.4	2.2
Fair Value Hedging [Member] | Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member]
Derivative Liability, Fair Value	8.9	7.5
Derivative, Collateral, Right to Reclaim Securities	(4.0)	(3.4)
Derivative, Collateral, Right to Reclaim Cash	(4.6)	(4.1)
Liability position offset by the asset position of counterparties with master netting agreements	0.3	0.0
Fair Value Hedging [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	9.0	7.5
Derivative, Collateral, Right to Reclaim Securities	(4.0)	(3.4)
Derivative, Collateral, Right to Reclaim Cash	(4.6)	(4.1)
Liability position offset by the asset position of counterparties with master netting agreements	0.4	0.0
Fair Value Hedging [Member] | Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Asset [Member]
Derivative Asset, Fair Value	5.7	3.5
Derivative, Collateral, Obligation to Return Securities	(5.2)	(0.9)
Derivative, Collateral, Obligation to Return Cash	0.0	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	0.5	2.6
Fair Value Hedging [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	5.8	3.6
Derivative, Collateral, Obligation to Return Securities	(5.2)	(0.9)
Derivative, Collateral, Obligation to Return Cash	0.0	0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral	0.6	2.7
Fair Value Hedging [Member] | Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Credit [Member]
Derivative Liability, Fair Value	6.5	0.9
Derivative, Collateral, Right to Reclaim Securities	(5.2)	(0.9)
Derivative, Collateral, Right to Reclaim Cash	(1.0)	0.0
Liability position offset by the asset position of counterparties with master netting agreements	0.3	0.0
Fair Value Hedging [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	6.5	1.0
Derivative, Collateral, Right to Reclaim Securities	(5.2)	(0.9)
Derivative, Collateral, Right to Reclaim Cash	(1.0)	0.0
Liability position offset by the asset position of counterparties with master netting agreements	$ 0.3	0.1
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		0.0
Derivative, Collateral, Right to Reclaim Cash		(0.1)
Liability position offset by the asset position of counterparties with master netting agreements		0.0
Fair Value Hedging [Member] | Natural Gas [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative Liability, Fair Value		0.1
Derivative, Collateral, Right to Reclaim Securities		0.0
Derivative, Collateral, Right to Reclaim Cash		(0.1)
Liability position offset by the asset position of counterparties with master netting agreements		$ 0.0