Contractual Obligations, Commercial Commitments and Contingenciesl (Schedule Of Contractual Obligations And Commercial Commitments) (Details) $ in Millions	Dec. 31, 2017 USD ($)
Electricity Purchase Commitments [Member]
Contractual Obligations, Commercial Commitments And Contingencies [Line Items]
Electricity Purchase Commitments	$ 370.9
Electricity Purchase Agreements Less Than 1 Year	178.5
Electricity Purchase Agreements in Years 2 and 3	171.2
Electricity Purchase Agreements in Years 4 and 5	21.2
Electricity Purchase Agreements, After Year 5	0.0
Electricity Purchase Commitments [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Contractual Obligations, Commercial Commitments And Contingencies [Line Items]
Electricity Purchase Commitments	370.9
Electricity Purchase Agreements Less Than 1 Year	178.5
Electricity Purchase Agreements in Years 2 and 3	171.2
Electricity Purchase Agreements in Years 4 and 5	21.2
Electricity Purchase Agreements, After Year 5	0.0
Coal Contracts [Member]
Contractual Obligations, Commercial Commitments And Contingencies [Line Items]
Total Coal Contracts	54.9
Coal Contracts, Less than 1 year	54.9
Coal Contracts, 2 - 3 years	0.0
Coal Contracts, 4 - 5 years	0.0
Coal Contracts, More than 5 years	0.0
Other Intangible Assets [Member]
Contractual Obligations, Commercial Commitments And Contingencies [Line Items]
Total Purchase orders and other contractual obligations	73.0
Purchase orders and other contractual obligations, Less than 1 year	18.9
Purchase orders and other contractual obligations, 2 - 3 years	27.1
Purchase orders and other contractual obligations, 4 - 5 years	27.0
Purchase orders and other contractual obligations, More than 5 years	0.0
Other Intangible Assets [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Contractual Obligations, Commercial Commitments And Contingencies [Line Items]
Total Purchase orders and other contractual obligations	73.0
Purchase orders and other contractual obligations, Less than 1 year	18.9
Purchase orders and other contractual obligations, 2 - 3 years	27.1
Purchase orders and other contractual obligations, 4 - 5 years	27.0
Purchase orders and other contractual obligations, More than 5 years	$ 0.0