Fair Value Fair Value Measurements (Fair Value of Assets and Liabilities Measured on a Non-recurring Basis) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 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	$ 1,659.6		1,659.6		$ 1,767.2
Impairment of Long-Lived Assets Held-for-use	0.0	$ 235.5	66.4	$ 235.5
Peaking Generating Facilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property, Plant and Equipment, Net		9.9		9.9
Impairment of Long-Lived Assets Held-for-use				4.7
Peaking Generating Facilities [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		0.0
Peaking Generating Facilities [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		0.0
Peaking Generating Facilities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property, Plant, and Equipment, Fair Value Disclosure		5.2		5.2
Killen [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property, Plant and Equipment, Net	35.2	315.1	35.2	315.1
Impairment of Long-Lived Assets Held-for-use			27.3	230.8
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	0.0	0.0	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	0.0	0.0	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	84.3	7.9	84.3
Stuart [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property, Plant and Equipment, Net	42.4		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		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		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		3.3
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	1,334.5		1,334.5		$ 1,440.6
Impairment of Long-Lived Assets Held-for-use	0.0	857.1	66.3	857.1
THE DAYTON POWER AND LIGHT COMPANY [Member] | Zimmer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property, Plant and Equipment, Net		429.9		429.9
Impairment of Long-Lived Assets Held-for-use				318.9
THE DAYTON POWER AND LIGHT COMPANY [Member] | Zimmer [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		0.0
THE DAYTON POWER AND LIGHT COMPANY [Member] | Zimmer [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		0.0
THE DAYTON POWER AND LIGHT COMPANY [Member] | Zimmer [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property, Plant, and Equipment, Fair Value Disclosure		111.0		111.0
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	330.5	35.2	330.5
Impairment of Long-Lived Assets Held-for-use			27.3	246.2
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	0.0	0.0	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	0.0	0.0	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	84.3	7.9	84.3
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	456.4	42.3	456.4
Impairment of Long-Lived Assets Held-for-use			39.0	292.0
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	0.0	0.0	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	0.0	0.0	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	$ 164.4	$ 3.3	$ 164.4
Income Approach Valuation Technique [Member] | Weighted Average [Member] | Peaking Generating Facilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent				(4.00%)
Fair Value Inputs, Discount Rate				7.00%
Fair Value Inputs, Long-term Revenue Growth Rate				(3.00%)
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%	6.00%
Fair Value Inputs, Discount Rate			7.00%	11.00%
Fair Value Inputs, Long-term Revenue Growth Rate				2.00%
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] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Zimmer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent				4.00%
Fair Value Inputs, Discount Rate				9.00%
Fair Value Inputs, Long-term Revenue Growth Rate				1.00%
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%	6.00%
Fair Value Inputs, Discount Rate			7.00%	11.00%
Fair Value Inputs, Long-term Revenue Growth Rate				2.00%
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%	5.00%
Fair Value Inputs, Discount Rate			7.00%	9.00%
Fair Value Inputs, Long-term Revenue Growth Rate				2.00%
Income Approach Valuation Technique [Member] | Minimum [Member] | Peaking Generating Facilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent				(29.00%)
Fair Value Inputs, Long-term Revenue Growth Rate				(22.00%)
Income Approach Valuation Technique [Member] | Minimum [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				(50.00%)
Fair Value Inputs, Long-term Revenue Growth Rate				(11.00%)
Income Approach Valuation Technique [Member] | Minimum [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Zimmer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent				(46.00%)
Fair Value Inputs, Long-term Revenue Growth Rate				(14.00%)
Income Approach Valuation Technique [Member] | Minimum [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				(50.00%)
Fair Value Inputs, Long-term Revenue Growth Rate				(11.00%)
Income Approach Valuation Technique [Member] | Minimum [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				(29.00%)
Fair Value Inputs, Long-term Revenue Growth Rate				(9.00%)
Income Approach Valuation Technique [Member] | Maximum [Member] | Peaking Generating Facilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent				24.00%
Fair Value Inputs, Long-term Revenue Growth Rate				17.00%
Income Approach Valuation Technique [Member] | Maximum [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				67.00%
Fair Value Inputs, Long-term Revenue Growth Rate				13.00%
Income Approach Valuation Technique [Member] | Maximum [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Zimmer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs, Long-term Pre-tax Operating Margin, Percent				80.00%
Fair Value Inputs, Long-term Revenue Growth Rate				13.00%
Income Approach Valuation Technique [Member] | Maximum [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				67.00%
Fair Value Inputs, Long-term Revenue Growth Rate				13.00%
Income Approach Valuation Technique [Member] | Maximum [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				52.00%
Fair Value Inputs, Long-term Revenue Growth Rate				10.00%
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			$ 2.4	$ 2.3
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			$ 1.6	$ 2.3