Fair Value Fair Value Measurements (Fair Value of Assets and Liabilities Measured on a Non-recurring Basis) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Increase (Decrease) in Asset Retirement Obligations	$ 4.4
Property, Plant and Equipment, Net	1,700.6		$ 1,767.2
Impairment of Long-Lived Assets Held-for-use	66.4	$ 0.0
Stuart [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property, Plant and Equipment, Net	42.4
Impairment of Long-Lived Assets Held-for-use	39.1
Stuart [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property, Plant, and Equipment, Fair Value Disclosure	0.0
Stuart [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property, Plant, and Equipment, Fair Value Disclosure	0.0
Stuart [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property, Plant, and Equipment, Fair Value Disclosure	3.3
Killen [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property, Plant and Equipment, Net	35.2
Impairment of Long-Lived Assets Held-for-use	27.3
Killen [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property, Plant, and Equipment, Fair Value Disclosure	0.0
Killen [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property, Plant, and Equipment, Fair Value Disclosure	0.0
Killen [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property, Plant, and Equipment, Fair Value Disclosure	7.9
THE DAYTON POWER AND LIGHT COMPANY [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Increase (Decrease) in Asset Retirement Obligations	4.4
Property, Plant and Equipment, Net	1,370.8		$ 1,440.6
Impairment of Long-Lived Assets Held-for-use	66.3	0.0
THE DAYTON POWER AND LIGHT COMPANY [Member] | Stuart [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property, Plant and Equipment, Net	42.3
Impairment of Long-Lived Assets Held-for-use	39.0
THE DAYTON POWER AND LIGHT COMPANY [Member] | Stuart [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property, Plant, and Equipment, Fair Value Disclosure	0.0
THE DAYTON POWER AND LIGHT COMPANY [Member] | Stuart [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property, Plant, and Equipment, Fair Value Disclosure	0.0
THE DAYTON POWER AND LIGHT COMPANY [Member] | Stuart [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property, Plant, and Equipment, Fair Value Disclosure	3.3
THE DAYTON POWER AND LIGHT COMPANY [Member] | Killen [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property, Plant and Equipment, Net	35.2
Impairment of Long-Lived Assets Held-for-use	27.3
THE DAYTON POWER AND LIGHT COMPANY [Member] | Killen [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property, Plant, and Equipment, Fair Value Disclosure	0.0
THE DAYTON POWER AND LIGHT COMPANY [Member] | Killen [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property, Plant, and Equipment, Fair Value Disclosure	0.0
THE DAYTON POWER AND LIGHT COMPANY [Member] | Killen [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property, Plant, and Equipment, Fair Value Disclosure	$ 7.9
Income Approach Valuation Technique [Member] | Weighted Average [Member] | Stuart [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent	10.00%
Fair Value Inputs, Discount Rate	7.00%
Income Approach Valuation Technique [Member] | Weighted Average [Member] | Killen [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent	22.00%
Fair Value Inputs, Discount Rate	7.00%
Income Approach Valuation Technique [Member] | Weighted Average [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Stuart [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent	10.00%
Fair Value Inputs, Discount Rate	7.00%
Income Approach Valuation Technique [Member] | Weighted Average [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Killen [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent	22.00%
Fair Value Inputs, Discount Rate	7.00%
Ash ponds, asbestos, river structures and underground storage tanks [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Increase (Decrease) in Asset Retirement Obligations	$ (3.1)	(0.6)
Ash ponds, asbestos, river structures and underground storage tanks [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Increase (Decrease) in Asset Retirement Obligations	$ (2.9)	$ (0.6)