Fixed-asset Impairment (Narrative) (Details) $ in Millions	3 Months Ended	6 Months Ended		12 Months Ended
	Mar. 31, 2014 USD ($) MW	Dec. 31, 2016 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Public Utility, Property, Plant and Equipment [Line Items]
Fixed-asset impairment (Note 15)		$ 623.5		$ 859.0	$ 0.0	$ 11.5
THE DAYTON POWER AND LIGHT COMPANY [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fixed-asset impairment (Note 15)		496.4		1,353.5	$ 0.0	0.0
Zimmer [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fixed-asset impairment (Note 15)				144.7
Zimmer [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fixed-asset impairment (Note 15)		91.3	$ 318.9	318.9
Conesville [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fixed-asset impairment (Note 15)				23.9
Conesville [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fixed-asset impairment (Note 15)				20.8
Hutchings Peakers [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fixed-asset impairment (Note 15)				1.6
Hutchings Peakers [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fixed-asset impairment (Note 15)				1.4
Stuart [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fixed-asset impairment (Note 15)				228.5
Stuart [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fixed-asset impairment (Note 15)		149.9	292.0	292.0
Killen [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fixed-asset impairment (Note 15)		75.4	230.8
Killen [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fixed-asset impairment (Note 15)		75.3	246.2	246.2
Peaking Generating Facilities [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fixed-asset impairment (Note 15)			4.7
Miami Fort [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fixed-asset impairment (Note 15)				149.4
Miami Fort [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fixed-asset impairment (Note 15)				157.7
East Bend Station [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fixed-asset impairment (Note 15)	$ 11.5			0.0		11.5
Fair Value						2.7
East Bend Station [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Production Plan Capacity | MW	186
Fair Value, Inputs, Level 3 [Member] | Zimmer [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fair Value		23.7		23.7
Fair Value, Inputs, Level 3 [Member] | Zimmer [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fair Value		23.7	111.0	23.7
Fair Value, Inputs, Level 3 [Member] | Conesville [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fair Value		1.1		1.1
Fair Value, Inputs, Level 3 [Member] | Conesville [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fair Value		1.1		1.1
Fair Value, Inputs, Level 3 [Member] | Hutchings Peakers [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fair Value		1.6		1.6
Fair Value, Inputs, Level 3 [Member] | Hutchings Peakers [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fair Value		1.6		1.6
Fair Value, Inputs, Level 3 [Member] | Stuart [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fair Value		57.4		57.4
Fair Value, Inputs, Level 3 [Member] | Stuart [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fair Value		57.4	164.4	57.4
Fair Value, Inputs, Level 3 [Member] | Killen [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fair Value		42.8	84.3	42.8
Fair Value, Inputs, Level 3 [Member] | Killen [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fair Value		42.8	84.3	42.8
Fair Value, Inputs, Level 3 [Member] | Peaking Generating Facilities [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fair Value			$ 5.2
Fair Value, Inputs, Level 3 [Member] | Miami Fort [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fair Value		36.5		36.5
Fair Value, Inputs, Level 3 [Member] | Miami Fort [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fair Value		$ 36.5		$ 36.5
Fair Value, Inputs, Level 3 [Member] | East Bend Station [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Fair Value						$ 2.7