Summary of Significant Accounting Policies (Narrative) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2020 USD ($) mi² employee customer generating_facility segment	Mar. 31, 2019 USD ($)	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	$ 39.6		$ 36.5
Entity number of employees | employee	625
Employees under a collective bargaining agreement which expires in October-2011	58.00%
Excise taxes collected	$ 12.4	$ 13.9
Restricted Cash and Cash Equivalents, Current	14.6		10.5
Restricted Cash and Cash Equivalents	54.2	112.8	47.0	$ 111.7
Public Utilities, Property, Plant and Equipment, Generation or Processing	$ 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	$ 7.3		10.8
Approximate number of retail customers | customer	527,000
Service area, square miles | mi²	6,000
Number of Operating Segments | segment	1
Entity number of employees | employee	622
Employees under a collective bargaining agreement which expires in October-2011	58.00%
Number Of Generating Facilities | generating_facility	1
Excise taxes collected	$ 12.4	13.9
Restricted Cash and Cash Equivalents, Current	14.6		10.5
Restricted Cash and Cash Equivalents	21.9	$ 60.8	$ 21.3	$ 66.2
Accounting Standards Update 2016-13 [Member]
Significant Accounting Policies [Line Items]
Accounts Receivable, before Allowance for Credit Loss, Current	61.6
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	$ 60.4