Summary of Significant Accounting Policies (Narrative) (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($) employee	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) mi² employee segment customer	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Significant Accounting Policies [Line Items]
Number of reportable segments | segment			1
Cash and Cash Equivalents, at Carrying Value	$ 412.2		$ 412.2		$ 36.5
Entity number of employees | employee	644		644
Employees under a collective bargaining agreement which expires in October-2011			58.00%
Excise taxes collected	$ 10.9	$ 10.4	$ 23.3	$ 24.3
Restricted Cash and Cash Equivalents, Current	0.1		0.1		10.5
Restricted Cash and Cash Equivalents	412.3	42.6	$ 412.3	42.6	47.0	$ 111.7
THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Number of reportable segments | segment			1
Cash and Cash Equivalents, at Carrying Value	$ 74.1		$ 74.1		10.8
Approximate number of retail customers | customer			528,000
Service area, square miles | mi²			6,000
Number of Operating Segments | segment			1
Entity number of employees | employee	643		643
Employees under a collective bargaining agreement which expires in October-2011			58.00%
Excise taxes collected	$ 10.9	10.4	$ 23.3	24.3
Restricted Cash and Cash Equivalents, Current	0.1		0.1		10.5
Restricted Cash and Cash Equivalents	$ 74.2	$ 17.1	$ 74.2	$ 17.1	$ 21.3	$ 66.2