Summary of Significant Accounting Policies (Narrative) (Details) $ in Millions		3 Months Ended		9 Months Ended
	Jan. 01, 2018 USD ($)	Sep. 30, 2018 USD ($) employee	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) mi² employee customer generating_facility segment	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Significant Accounting Policies [Line Items]
Number of reportable segments | segment				1
Cash and Cash Equivalents, at Carrying Value		$ 65.4		$ 65.4		$ 24.5
Entity number of employees | employee		674		674
Employees under a collective bargaining agreement which expires in October-2011				53.00%
Excise taxes collected		$ 13.8	$ 13.0	$ 39.2	$ 36.9
Restricted Cash and Cash Equivalents, Current		22.0		22.0		0.4
Restricted Cash and Cash Equivalents		87.4	23.4	87.4	23.4	24.9	$ 54.6
AOCI reclassed to Retained Earnings before tax	$ 1.6
AOCI reclassed to Retained Earnings, net of tax	1.0	0.0	0.0	$ (1.0)	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		$ 10.7		$ 10.7		5.2
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		648		648
Employees under a collective bargaining agreement which expires in October-2011				55.00%
Number Of Generating Facilities | generating_facility				1
Excise taxes collected		$ 13.8	13.0	$ 39.2	36.9
Restricted Cash and Cash Equivalents, Current		2.0		2.0		0.4
Restricted Cash and Cash Equivalents		12.7	15.3	12.7	15.3	$ 5.6	$ 1.6
AOCI reclassed to Retained Earnings before tax	1.7
AOCI reclassed to Retained Earnings, net of tax	$ 1.1	$ 0.0	$ 0.0	(1.1)	0.0
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
Non-service Pension Costs					(1.6)
Increase (Decrease) in Restricted Cash					27.0
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
AOCI reclassed to Retained Earnings, net of tax				$ 1.1
Non-service Pension Costs					1.2
Increase (Decrease) in Restricted Cash					$ 27.0