Summary of Significant Accounting Policies (Narrative) (Details) $ in Millions	1 Months Ended	3 Months Ended		6 Months Ended
	Jan. 31, 2016	Jun. 30, 2016 USD ($) employee	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($) mi² employee segment generating_facility customer	Jun. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)
Significant Accounting Policies [Line Items]
Number of reportable segments | segment				1
Entity number of employees | employee		1,169		1,169
Employees under a collective bargaining agreement which expires in October-2011				62.00%
Excise taxes collected		$ 11.6	$ 11.5	$ 24.5	$ 25.5
THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Approximate number of retail customers | customer				517,000
Service area, square miles | mi²				6,000
Entity number of employees | employee		1,161		1,161
Employees under a collective bargaining agreement which expires in October-2011				61.00%
Number of generating facilities | generating_facility				5
Excise taxes collected		$ 11.6	$ 11.5	$ 24.5	$ 25.5
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%