Summary of Significant Accounting Policies (Narrative) (Details) mi² in Thousands, customer in Thousands, $ in Millions	1 Months Ended	3 Months Ended
	Jan. 31, 2016	Mar. 31, 2016 USD ($) mi² employee segment generating_facility customer	Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($)
Significant Accounting Policies [Line Items]
Number of reportable segments | segment		1
Entity number of employees | employee		1,179
Employees under a collective bargaining agreement which expires in October-2011		62.00%
Excise taxes collected		$ 12.9	$ 14.0
THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Approximate number of retail customers | customer		518
Service area, square miles | mi²		6
Entity number of employees | employee		1,172
Employees under a collective bargaining agreement which expires in October-2011		62.00%
Number of generating facilities | generating_facility		5
Excise taxes collected		$ 12.9	$ 14.0
Line of Credit [Member]
Significant Accounting Policies [Line Items]
Noncurrent portion of deferred finance costs				$ 3.1
Line of Credit [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Noncurrent portion of deferred finance costs				0.7
Adjustments for New Accounting Pronouncement [Member]
Significant Accounting Policies [Line Items]
Current portion of deferred finance costs				(2.1)
Noncurrent portion of deferred finance costs				(14.0)
Adjustments for New Accounting Pronouncement [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Current portion of deferred finance costs				(1.8)
Noncurrent portion of deferred finance costs				$ (4.5)
Year 2016 [Member]
Significant Accounting Policies [Line Items]
Electric generation through competitive bid	100.00%