Derivative Instruments and Hedging Activities (Fair Value and Balance Sheet Location (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Derivative Liability, Fair Value	$ 18.7
Liability position offset by the asset position of counterparties with master netting agreements	9.6
THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative Liability, Fair Value	18.9
Liability position offset by the asset position of counterparties with master netting agreements	9.6
Total Assets [Member]
Derivative Asset, Fair Value	19.7	$ 14.9
Derivative, Collateral, Obligation to Return Securities	(9.6)	(6.6)
Derivative Asset, Fair Value, Amount Offset Against Collateral	10.1	8.3
Total Assets [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative Asset, Fair Value	19.9	15.1
Derivative, Collateral, Obligation to Return Securities	(9.6)	(6.6)
Derivative Asset, Fair Value, Amount Offset Against Collateral	10.3	8.5
Total Liabilities [Member]
Derivative Liability, Fair Value	18.7	12.2
Derivative, Collateral, Right to Reclaim Securities	(9.6)	(6.6)
Derivative, Collateral, Right to Reclaim Cash	(7.4)	(4.9)
Liability position offset by the asset position of counterparties with master netting agreements	1.7	0.7
Total Liabilities [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative Liability, Fair Value	18.9	12.3
Derivative, Collateral, Right to Reclaim Securities	(9.6)	(6.6)
Derivative, Collateral, Right to Reclaim Cash	(7.4)	(4.9)
Liability position offset by the asset position of counterparties with master netting agreements	1.9	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	6.8	5.6
Derivative, Collateral, Obligation to Return Securities	(4.1)	(2.0)
Derivative Asset, Fair Value, Amount Offset Against Collateral	2.7	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	6.8	5.6
Derivative, Collateral, Obligation to Return Securities	(4.1)	(2.0)
Derivative Asset, Fair Value, Amount Offset Against Collateral	2.7	3.6
Cash Flow Hedging [Member] | Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member]
Derivative Liability, Fair Value	5.3	2.1
Derivative, Collateral, Right to Reclaim Securities	(4.1)	(2.0)
Liability position offset by the asset position of counterparties with master netting agreements	1.2	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	5.3	2.1
Derivative, Collateral, Right to Reclaim Securities	(4.1)	(2.0)
Liability position offset by the asset position of counterparties with master netting agreements	1.2	0.1
Cash Flow Hedging [Member] | Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Asset [Member]
Derivative Asset, Fair Value	2.7	0.3
Derivative, Collateral, Obligation to Return Securities	(1.4)	(0.3)
Derivative Asset, Fair Value, Amount Offset Against Collateral	1.3
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	2.7	0.3
Derivative, Collateral, Obligation to Return Securities	(1.4)	(0.3)
Derivative Asset, Fair Value, Amount Offset Against Collateral	1.3
Cash Flow Hedging [Member] | Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Credit [Member]
Derivative Liability, Fair Value	1.4	0.6
Derivative, Collateral, Right to Reclaim Securities	(1.4)	(0.3)
Derivative, Collateral, Right to Reclaim Cash		(0.3)
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	1.4	0.6
Derivative, Collateral, Right to Reclaim Securities	(1.4)	(0.3)
Derivative, Collateral, Right to Reclaim Cash		(0.3)
Fair Value Hedging [Member] | Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Prepayments and Current Assets [Member]
Derivative Asset, Fair Value	4.3	5.5
Derivative, Collateral, Obligation to Return Securities	(2.3)	(3.4)
Derivative Asset, Fair Value, Amount Offset Against Collateral	2.0	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	4.4	5.6
Derivative, Collateral, Obligation to Return Securities	(2.3)	(3.4)
Derivative Asset, Fair Value, Amount Offset Against Collateral	2.1	2.2
Fair Value Hedging [Member] | Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member]
Derivative Liability, Fair Value	9.6	7.5
Derivative, Collateral, Right to Reclaim Securities	(2.3)	(3.4)
Derivative, Collateral, Right to Reclaim Cash	(7.3)	(4.1)
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 Current Liabilities [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative Liability, Fair Value	9.7	7.5
Derivative, Collateral, Right to Reclaim Securities	(2.3)	(3.4)
Derivative, Collateral, Right to Reclaim Cash	(7.3)	(4.1)
Liability position offset by the asset position of counterparties with master netting agreements	0.1
Fair Value Hedging [Member] | Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Asset [Member]
Derivative Asset, Fair Value	5.5	3.5
Derivative, Collateral, Obligation to Return Securities	(1.4)	(0.9)
Derivative Asset, Fair Value, Amount Offset Against Collateral	4.1	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	5.6	3.6
Derivative, Collateral, Obligation to Return Securities	(1.4)	(0.9)
Derivative Asset, Fair Value, Amount Offset Against Collateral	4.2	2.7
Fair Value Hedging [Member] | Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Credit [Member]
Derivative Liability, Fair Value	1.4	0.9
Derivative, Collateral, Right to Reclaim Securities	(1.4)	(0.9)
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	1.5	1.0
Derivative, Collateral, Right to Reclaim Securities	(1.4)	(0.9)
Liability position offset by the asset position of counterparties with master netting agreements	0.1	0.1
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 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)
Fair Value Hedging [Member] | Commodity Contract - FTR [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member]
Derivative Liability, Fair Value	0.9	0.6
Derivative, Collateral, Right to Reclaim Securities	(0.4)
Liability position offset by the asset position of counterparties with master netting agreements	0.5	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	0.9	0.6
Derivative, Collateral, Right to Reclaim Securities	(0.4)
Liability position offset by the asset position of counterparties with master netting agreements	0.5	0.6
Fair Value Hedging [Member] | Commodity Contract - Heating Oil [Member] | Short-term Derivative Positions [Member] | Other Prepayments and Current Assets [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative Asset, Fair Value, Amount Offset Against Collateral	0.0
Fair Value Hedging [Member] | Commodity Contract - Heating Oil [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member]
Derivative Liability, Fair Value	0.1	0.4
Derivative, Collateral, Right to Reclaim Cash	(0.1)	(0.4)
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	0.1	0.4
Derivative, Collateral, Right to Reclaim Cash	$ (0.1)	(0.4)
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 Cash		(0.1)
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 Cash		$ (0.1)