Property, Plant and Equipment (Changes in the Liability for Generation of AROs) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at January 1	$ 138.8	$ 65.9
Additions	0.1	70.2
Asset Retirement Obligation, Revision of Estimate	(6.3)
Accretion expense	3.7	2.7
Settlements	(0.1)	0.0
Balance at December 31	131.2	138.8
Asset Retirement Obligation, Reduction Due to Plants Sold or Held-for-sale	(5.0)
THE DAYTON POWER AND LIGHT COMPANY [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at January 1	8.2	5.0
Additions		2.7
Accretion expense	0.1	0.3
Settlements	(0.3)	0.2
Balance at December 31	$ 8.0	$ 8.2