Summary of Significant Accounting Policies (Narrative) (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($) mi² employee segment customer	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) employee segment generating_facility	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Significant Accounting Policies [Line Items]
Number of reportable segments | segment	1
Cash and Cash Equivalents, at Carrying Value	$ 31.1		$ 31.1		$ 90.5
Entity number of employees | employee	661		661
Employees under a collective bargaining agreement which expires in October-2011			56.00%
Excise taxes collected	$ 10.4	$ 12.3	$ 24.3	$ 25.4
Restricted Cash and Cash Equivalents, Current	11.5		11.5		21.2
Restricted Cash and Cash Equivalents	42.6	68.4	42.6	68.4	111.7	$ 24.9
Public Utilities, Property, Plant and Equipment, Generation or Processing	0.0		$ 0.0
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	$ 5.6		$ 5.6		45.0
Approximate number of retail customers | customer	525,000
Service area, square miles | mi²	6,000
Number of Operating Segments | segment			1
Entity number of employees | employee	649		649
Employees under a collective bargaining agreement which expires in October-2011			57.00%
Number Of Generating Facilities | generating_facility			1
Excise taxes collected	$ 10.4	12.3	$ 24.3	25.4
Restricted Cash and Cash Equivalents, Current	11.5		11.5		21.2
Restricted Cash and Cash Equivalents	$ 17.1	$ 30.2	$ 17.1	$ 30.2	$ 66.2	$ 5.6