Summary of Significant Accounting Policies (Narrative) (Details) $ in Millions	1 Months Ended	3 Months Ended		6 Months Ended
	Jan. 31, 2016	Jun. 30, 2017 USD ($) employee	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) mi² employee segment generating_facility customer	Jun. 30, 2016 USD ($)
Significant Accounting Policies [Line Items]
Number of reportable segments | segment				2
Entity number of employees | employee		1,115		1,115
Employees under a collective bargaining agreement which expires in October-2011				60.00%
Excise taxes collected | $		$ 11.4	$ 11.6	$ 23.9	$ 24.5
THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Approximate number of retail customers | customer				520,000
Service area, square miles | mi²				6,000
Number of Operating Segments | segment				2
Entity number of employees | employee		1,106		1,106
Employees under a collective bargaining agreement which expires in October-2011				61.00%
Number Of Generating Facilities | generating_facility				5
Excise taxes collected | $		$ 11.4	$ 11.6	$ 23.9	$ 24.5
Year 2016 [Member]
Significant Accounting Policies [Line Items]
Electric generation through competitive bid	100.00%