Income Taxes (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Entity Information [Line Items]
Effective income tax rates	21.80%	45.70%	36.90%	38.60%
Estimated annual effective income tax rate			36.00%	38.60%
Change in effective tax rate after adjustment for flow-through depreciation	10.80%
THE DAYTON POWER AND LIGHT COMPANY [Member]
Entity Information [Line Items]
Effective income tax rates	28.90%	39.60%	5.30%	36.70%
Estimated annual effective income tax rate			19.00%	36.70%
Income Taxes Paid, Net			$ 22.2	$ 0.3
Change in effective tax rate after adjustment for flow-through depreciation	8.15%