Summary of Significant Accounting Policies (Narrative) (Details) $ in Millions	1 Months Ended	3 Months Ended
	Jan. 31, 2016	Mar. 31, 2017 USD ($) mi² employee segment generating_facility customer	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)
Significant Accounting Policies [Line Items]
Number of reportable segments | segment		2
Entity number of employees | employee		1,153
Employees under a collective bargaining agreement which expires in October-2011		62.00%
Excise taxes collected		$ 12.5	$ 12.9
THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Approximate number of retail customers | customer		520,000
Service area, square miles | mi²		6,000
Number of Operating Segments | segment		2
Entity number of employees | employee		1,145
Employees under a collective bargaining agreement which expires in October-2011		62.00%
Number Of Generating Facilities | generating_facility		5
Excise taxes collected		$ 12.5	$ 12.9
Line of Credit [Member]
Significant Accounting Policies [Line Items]
Noncurrent portion of deferred finance costs
Line of Credit [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Noncurrent portion of deferred finance costs
Adjustments for New Accounting Pronouncement [Member]
Significant Accounting Policies [Line Items]
Current portion of deferred finance costs
Noncurrent portion of deferred finance costs
Adjustments for New Accounting Pronouncement [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Current portion of deferred finance costs
Noncurrent portion of deferred finance costs
Year 2016 [Member]
Significant Accounting Policies [Line Items]
Electric generation through competitive bid	100.00%