Income Taxes (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Entity Information [Line Items]
Effective income tax rates	17.60%	37.90%
Estimated annual effective income tax rate	19.20%	37.60%
Federal Income Tax Rate for Corporations	21.00%	35.00%
Non-cash capital contribution	$ 45.1	$ 0.0
THE DAYTON POWER AND LIGHT COMPANY [Member]
Entity Information [Line Items]
Effective income tax rates	18.70%	32.00%
Estimated annual effective income tax rate	19.80%	33.80%
Federal Income Tax Rate for Corporations	21.00%	35.00%