Property, Plant and Equipment with Corresponding Depreciation Rates (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Public Utility, Property, Plant and Equipment [Line Items]
Total property, plant and equipment in service	$ 1,554.7	$ 1,985.6
Total property, plant and equipment in service, Composite Rate	5.00%	6.10%	4.40%
Regulated Operation [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Transmission	$ 242.7	$ 247.3
Distribution	1,197.5	1,141.1
General	13.7	13.7
Non-depreciable	64.7	63.5
Total property, plant and equipment in service	$ 1,518.6	$ 1,465.6
Transmission, Composite Rate	4.00%	3.90%
Distribution, Composite Rate	4.90%	4.70%
General, Composite Rate	7.10%	7.40%
Unregulated Operation [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Non-depreciable	$ 5.8	$ 19.8
Total property, plant and equipment in service	36.1	520.0
Production / Generation	$ 10.9	$ 483.2
Production/Generation, Composite Rate	63.20%	11.70%
Other	$ 19.4	$ 17.0
Other, Composite Rate	7.00%	8.00%
THE DAYTON POWER AND LIGHT COMPANY [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Total property, plant and equipment in service	$ 2,247.2	$ 2,213.5
Total property, plant and equipment in service, Composite Rate	3.40%	4.60%	2.50%
THE DAYTON POWER AND LIGHT COMPANY [Member] | Regulated Operation [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Transmission	$ 414.6	$ 421.1
Distribution	1,735.9	1,693.5
General	31.2	31.6
Non-depreciable	64.6	63.5
Total property, plant and equipment in service	$ 2,246.3	$ 2,209.7
Transmission, Composite Rate	2.40%	2.30%
Distribution, Composite Rate	3.40%	3.20%
General, Composite Rate	3.10%	3.20%
THE DAYTON POWER AND LIGHT COMPANY [Member] | Unregulated Operation [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Non-depreciable	$ 0.9	$ 3.8
Production / Generation	$ 0.2	$ 0.3
Production/Generation, Composite Rate	2.70%	2.70%
Other	$ 0.7	$ 3.5