Summary of Significant Accounting Policies (Narrative) (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($) employee segment	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) mi² employee customer generating_facility segment	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Significant Accounting Policies [Line Items]
Number of reportable segments | segment	1
Cash and Cash Equivalents, at Carrying Value	$ 36.8		$ 36.8		$ 90.5
Entity number of employees | employee	653		653
Employees under a collective bargaining agreement which expires in October-2011			57.00%
Excise taxes collected	$ 13.0	$ 13.8	$ 37.3	$ 39.2
Restricted Cash and Cash Equivalents, Current	17.6		17.6		21.2
Restricted Cash and Cash Equivalents	54.4	87.4	54.4	87.4	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	$ 14.4		$ 14.4		45.0
Approximate number of retail customers | customer			524,000
Service area, square miles | mi²			6,000
Number of Operating Segments | segment			1
Entity number of employees | employee	642		642
Employees under a collective bargaining agreement which expires in October-2011			58.00%
Number Of Generating Facilities | generating_facility			1
Excise taxes collected	$ 13.0	$ 13.8	$ 37.3	$ 39.2
Restricted Cash and Cash Equivalents, Current	17.6		17.6		21.2
Restricted Cash and Cash Equivalents	32.0		32.0		$ 66.2	$ 12.7	$ 5.6
Accounting Standards Update 2016-13 [Member]
Significant Accounting Policies [Line Items]
Accounts Receivable, before Allowance for Credit Loss, Current	70.2		70.2
Accounting Standards Update 2016-13 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Accounts Receivable, before Allowance for Credit Loss, Current	$ 72.1		$ 72.1