Shareholder's Equity (Details) - USD ($) $ / shares in Units, $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Class of Stock [Line Items]
Non-cash capital contribution	$ 30.2	$ 0.0
Common stock, shares authorized	1,500		1,500
Common stock, shares outstanding	1		1
THE DAYTON POWER AND LIGHT COMPANY [Member]
Class of Stock [Line Items]
Common stock, shares authorized	250,000,000		250,000,000
Par value common shares (in USD per share)	$ 0.01		$ 0.01
Common stock, shares outstanding	41,172,173		41,172,173
PUCO merger equity ratio approval (at least)	50.00%
PUCO Equity Ratio	42.00%
PUCO merger, maximum long-term debt allowed	$ 750.0
PUCO merger, maximum long-term debt as percent of rate base (percent)	75.00%
Long-term Debt, Gross	$ 594.9
Proceeds from Contributions from Parent	80.0	70.0
Payments of Ordinary Dividends, Common Stock	$ 43.8	$ 19.0