Subsequent Events (Details) - USD ($) $ in Millions		12 Months Ended
	Feb. 26, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Subsequent Event [Line Items]
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest		$ (119.9)	$ (793.3)	$ (231.4)
Property, Plant and Equipment, Additions		121.5	148.5	137.2
THE DAYTON POWER AND LIGHT COMPANY [Member]
Subsequent Event [Line Items]
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest		$ 88.5	$ 143.6	$ 189.0
Beckjord [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	$ (11.7)
Property, Plant and Equipment, Additions	15.0
Beckjord [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	(12.4)
Property, Plant and Equipment, Additions	$ 15.0