Summary of Significant Accounting Policies (Narrative) (Details) $ in Millions		3 Months Ended		6 Months Ended
	Jan. 01, 2018 USD ($)	Jun. 30, 2018 USD ($) employee	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) mi² employee customer generating_facility segment	Jun. 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		$ 61.2		$ 61.2		$ 24.5
Entity number of employees | employee		704		704
Employees under a collective bargaining agreement which expires in October-2011				52.00%
Excise taxes collected		$ 12.3	$ 11.4	$ 25.4	$ 23.9
Restricted Cash and Cash Equivalents, Current		7.2		7.2		0.4
Restricted Cash and Cash Equivalents		68.4	34.1	68.4	34.1	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	0.1	$ (1.0)	0.2
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		$ 23.0		$ 23.0		5.2
Approximate number of retail customers | customer				523,000
Service area, square miles | mi²				6,000
Number of Operating Segments | segment				1
Entity number of employees | employee		641		641
Employees under a collective bargaining agreement which expires in October-2011				55.00%
Number Of Generating Facilities | generating_facility				1
Excise taxes collected		$ 12.3	11.4	$ 25.4	23.9
Restricted Cash and Cash Equivalents, Current		7.2		7.2		0.4
Restricted Cash and Cash Equivalents		30.2	6.1	30.2	6.1	$ 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	$ 0.1	(1.1)	0.2
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					(1.0)
Increase (Decrease) in Restricted Cash					27.5
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					0.9
Increase (Decrease) in Restricted Cash					$ 27.5