Summary of Significant Accounting Policies (Narrative) (Details) mi² in Thousands, customer in Thousands	1 Months Ended	9 Months Ended
	Jan. 31, 2016	Sep. 30, 2015 mi² employee customer generating_facility segment
Significant Accounting Policies [Line Items]
Number of reportable segments | segment		2
Approximate number of competitive retail customers | customer		128
Entity number of employees		1,234
Employees under a collective bargaining agreement which expires in October-2011		59.00%
THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Approximate number of retail customers | customer		515
Service area, square miles | mi²		6
Number of generating facilities | generating_facility		5
Entity number of employees		1,194
Employees under a collective bargaining agreement which expires in October-2011		61.00%
Year 2015 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Electric generation through competitive bid		60.00%
Year 2016 [Member] | Scenario, Forecast [Member]
Significant Accounting Policies [Line Items]
Electric generation through competitive bid	100.00%