Fair Value Measurements (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Debt maturity date, earliest	2019
Debt maturity date, latest	2061
Unrealized gains and immaterial losses on Master Trust assets in AOCI	$ 1.6	$ 1.0
Unrealized gains and immaterial losses on Master Trust assets in AOCI, after tax	$ 1.0	0.6
Percent of inputs to the fair value of derivative instruments from quoted market prices	98.70%
Gross additions to our existing landfill and asbestos AROs	$ (2.6)	72.9
Gross additions to our existing landfill and asbestos AROs, after tax	(1.7)	47.4
Available-for-sale Securities, Gross Realized Gains (Losses), Sale Proceeds	0.9
AvailableForSaleSecuritiesGross Realized Gains Losses Sale Proceeds Net of Tax	0.6
THE DAYTON POWER AND LIGHT COMPANY [Member]
Unrealized gains and immaterial losses on Master Trust assets in AOCI	1.7	1.1
Unrealized gains and immaterial losses on Master Trust assets in AOCI, after tax	$ 1.1	0.7
Percent of inputs to the fair value of derivative instruments from quoted market prices	100.00%
Gross additions to our existing landfill and asbestos AROs	$ (0.2)	3.2
Gross additions to our existing landfill and asbestos AROs, after tax	(0.1)	$ 2.1
Available-for-sale Securities, Gross Realized Gains (Losses), Sale Proceeds	0.9
AvailableForSaleSecuritiesGross Realized Gains Losses Sale Proceeds Net of Tax	0.6
Stuart and Killen [Member]
Gross additions to our existing landfill and asbestos AROs	67.9
Gross additions to our existing landfill and asbestos AROs, after tax	$ 44.1