Summary of Significant Accounting Policies (Narrative) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($) mi² employee segment generating_facility customer	Mar. 31, 2018 USD ($)	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	$ 70.8		$ 90.5
Entity number of employees | employee	629
Employees under a collective bargaining agreement which expires in October-2011	58.00%
Excise taxes collected	$ 13.9	$ 13.1
Restricted Cash and Cash Equivalents, Current	42.0		21.2
Restricted Cash and Cash Equivalents	112.8	148.5	111.7	$ 24.9
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	$ 18.8		45.0
Approximate number of retail customers | customer	526,000
Service area, square miles | mi²	6,000
Number of Operating Segments | segment	1
Entity number of employees | employee	617
Employees under a collective bargaining agreement which expires in October-2011	59.00%
Number Of Generating Facilities | generating_facility	1
Excise taxes collected	$ 13.9	13.1
Restricted Cash and Cash Equivalents, Current	42.0		21.2
Restricted Cash and Cash Equivalents	60.8	1.9	$ 66.2	$ 5.6
Adjustments for New Accounting Pronouncement [Member]
Significant Accounting Policies [Line Items]
AOCI reclassed to Retained Earnings before tax		1.6
AOCI reclassed to Retained Earnings, net of tax		1.0
Adjustments for New Accounting Pronouncement [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
AOCI reclassed to Retained Earnings before tax	1.7	1.7
AOCI reclassed to Retained Earnings, net of tax	$ 1.1	1.1
Non-service Pension Costs		1.2
Increase (Decrease) in Restricted Cash		$ 27.0