Benefit Plans (Fair Value Measurements for Pension Plan Assets) (Details) - Pension [Member] - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Total Pension Plan Assets	$ 357.5	$ 341.0	$ 345.4
THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Pension Plan Assets	357.5	341.0	$ 345.4
Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Pension Plan Assets	321.3	307.9
Fair Value, Inputs, Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Pension Plan Assets	321.3	307.9
Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Pension Plan Assets	36.2	33.1
Fair Value, Inputs, Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Pension Plan Assets	36.2	33.1
Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Pension Plan Assets	0.0	0.0
Fair Value, Inputs, Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Pension Plan Assets	0.0	0.0
U.S. Equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Equity Securities	78.2	81.4
U.S. Equities [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Equity Securities	78.2	81.4
U.S. Equities [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Equity Securities	78.2	81.4
U.S. Equities [Member] | Fair Value, Inputs, Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Equity Securities	78.2	81.4
U.S. Equities [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Equity Securities	0.0	0.0
U.S. Equities [Member] | Fair Value, Inputs, Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Equity Securities	0.0	0.0
U.S. Equities [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Equity Securities	0.0	0.0
U.S. Equities [Member] | Fair Value, Inputs, Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Equity Securities	0.0	0.0
International Equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Equity Securities	46.3	44.4
International Equities [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Equity Securities	46.3	44.4
International Equities [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Equity Securities	46.3	44.4
International Equities [Member] | Fair Value, Inputs, Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Equity Securities	46.3	44.4
International Equities [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Equity Securities	0.0	0.0
International Equities [Member] | Fair Value, Inputs, Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Equity Securities	0.0	0.0
International Equities [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Equity Securities	0.0	0.0
International Equities [Member] | Fair Value, Inputs, Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Equity Securities	0.0	0.0
Fixed Income Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Equity Securities	163.3	151.1
Fixed Income Funds [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Equity Securities	163.3	151.1
Fixed Income Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Equity Securities	163.3	151.1
Fixed Income Funds [Member] | Fair Value, Inputs, Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Equity Securities	163.3	151.1
Fixed Income Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Equity Securities	0.0	0.0
Fixed Income Funds [Member] | Fair Value, Inputs, Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Equity Securities	0.0	0.0
Fixed Income Funds [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Equity Securities	0.0	0.0
Fixed Income Funds [Member] | Fair Value, Inputs, Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Equity Securities	0.0	0.0
US Treasury Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Debt Securities	33.5	31.0
US Treasury Securities [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Debt Securities	33.5	31.0
US Treasury Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Debt Securities	33.5	31.0
US Treasury Securities [Member] | Fair Value, Inputs, Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Debt Securities	33.5	31.0
US Treasury Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Debt Securities	0.0	0.0
US Treasury Securities [Member] | Fair Value, Inputs, Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Debt Securities	0.0	0.0
US Treasury Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Debt Securities	0.0	0.0
US Treasury Securities [Member] | Fair Value, Inputs, Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Debt Securities	0.0	0.0
Core Property Collective Fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Other Investments	36.2	33.1
Core Property Collective Fund [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Other Investments	36.2	33.1
Core Property Collective Fund [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Other Investments	0.0	0.0
Core Property Collective Fund [Member] | Fair Value, Inputs, Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Other Investments	0.0	0.0
Core Property Collective Fund [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Other Investments	36.2	33.1
Core Property Collective Fund [Member] | Fair Value, Inputs, Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Other Investments	36.2	33.1
Core Property Collective Fund [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Other Investments	0.0	0.0
Core Property Collective Fund [Member] | Fair Value, Inputs, Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Other Investments	$ 0.0	0.0
common collective [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Other Investments		0.0
common collective [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Other Investments		0.0
common collective [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Other Investments		0.0
common collective [Member] | Fair Value, Inputs, Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Other Investments		0.0
common collective [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Other Investments		0.0
common collective [Member] | Fair Value, Inputs, Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Other Investments		0.0
common collective [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Other Investments		0.0
common collective [Member] | Fair Value, Inputs, Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Other Investments		$ 0.0