Property, Plant and Equipment (Changes in the Liability for Generation of AROs) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at January 1	$ 65.9	$ 26.9
Additions	70.2	40.3
Accretion expense	2.7	1.9
Settlements	0.0	(3.2)
Balance at December 31	138.8	65.9
THE DAYTON POWER AND LIGHT COMPANY [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at January 1	62.1	22.9
Additions	70.2	40.3
Accretion expense	2.9	2.1
Settlements	0.0	(3.2)
Balance at December 31	$ 135.2	$ 62.1