Summary of Significant Accounting Policies (Narrative) (Details) $ in Millions		3 Months Ended
	Jan. 01, 2018 USD ($)	Mar. 31, 2018 USD ($) mi² employee customer generating_facility segment	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Significant Accounting Policies [Line Items]
Number of reportable segments | segment		2
Cash and Cash Equivalents, at Carrying Value		$ 123.9		$ 24.5
Entity number of employees | employee		1,051
Employees under a collective bargaining agreement which expires in October-2011		59.00%
Excise taxes collected		$ 13.1	$ 12.5
Restricted Cash and Cash Equivalents, Current		27.4		1.9
Restricted Cash and Cash Equivalents		151.3	62.7	26.4	$ 83.6
AOCI reclassed to Retained Earnings before tax		1.6
AOCI reclassed to Retained Earnings, net of tax		$ (1.0)
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		$ 1.0		5.2
Approximate number of retail customers | customer		523,000
Service area, square miles | mi²		6,000
Number of Operating Segments | segment		2
Entity number of employees | employee		673
Employees under a collective bargaining agreement which expires in October-2011		53.00%
Number Of Generating Facilities | generating_facility		3
Excise taxes collected		$ 13.1	12.5
Restricted Cash and Cash Equivalents, Current		0.9		0.4
Restricted Cash and Cash Equivalents		1.9	13.3	$ 5.6	$ 30.6
AOCI reclassed to Retained Earnings before tax	$ 1.7
AOCI reclassed to Retained Earnings, net of tax	$ 1.1	(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			3.6
Increase (Decrease) in Restricted Cash			20.6
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			0.8
Increase (Decrease) in Restricted Cash			$ 20.6