Fair Value Measurements (Fair Value of Assets and Liabilities Measured on a Nonrecurring Basis) (Details) - USD ($) $ in Millions		3 Months Ended				6 Months Ended		12 Months Ended
		Dec. 31, 2015	Mar. 31, 2015	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Sep. 30, 2015	Sep. 30, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value		$ 2,537.2				$ 1,767.2		$ 1,767.2	$ 2,537.2
Fixed-asset impairment (Note 15)						623.5		859.0	0.0	$ 11.5
Goodwill impairment	[1]							0.0	317.0	135.8
THE DAYTON POWER AND LIGHT COMPANY [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value		2,714.0				1,440.6		1,440.6	2,714.0
Fixed-asset impairment (Note 15)						496.4		1,353.5	0.0	0.0
Goodwill		0.0							0.0	317.0	$ 317.0
Goodwill impairment		$ 317.0	$ 317.0						$ 317.0
DPLER [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Goodwill					$ 135.8							$ 135.8
Goodwill impairment				$ 135.8	135.8
Killen [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value						118.2	$ 315.1	118.2
Fixed-asset impairment (Note 15)						75.4	230.8
Killen [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets held and used, fair value						0.0	0.0	0.0
Killen [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets held and used, fair value						0.0	0.0	0.0
Killen [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets held and used, fair value						42.8	84.3	42.8
Killen [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value						118.1	330.5	118.1
Fixed-asset impairment (Note 15)						75.3	246.2	246.2
Killen [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets held and used, fair value						42.8	84.3	42.8
Stuart [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value						285.9		285.9
Fixed-asset impairment (Note 15)								228.5
Stuart [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets held and used, fair value						0.0		0.0
Stuart [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets held and used, fair value						0.0		0.0
Stuart [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets held and used, fair value						57.4		57.4
Stuart [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value						207.3	456.4	207.3
Fixed-asset impairment (Note 15)						149.9	292.0	292.0
Stuart [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets held and used, fair value						57.4	164.4	57.4
Miami Fort [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value						185.9		185.9
Fixed-asset impairment (Note 15)								149.4
Miami Fort [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets held and used, fair value						0.0		0.0
Miami Fort [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets held and used, fair value						0.0		0.0
Miami Fort [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets held and used, fair value						36.5		36.5
Miami Fort [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value						194.2		194.2
Fixed-asset impairment (Note 15)								157.7
Miami Fort [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets held and used, fair value						36.5		36.5
Zimmer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value						168.4		168.4
Fixed-asset impairment (Note 15)								144.7
Zimmer [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets held and used, fair value						0.0		0.0
Zimmer [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets held and used, fair value						0.0		0.0
Zimmer [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets held and used, fair value						23.7		23.7
Zimmer [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value						115.0	429.9	115.0
Fixed-asset impairment (Note 15)						91.3	318.9	318.9
Zimmer [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets held and used, fair value						23.7	111.0	23.7
Conesville [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value						25.0		25.0
Fixed-asset impairment (Note 15)								23.9
Conesville [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets held and used, fair value						0.0		0.0
Conesville [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets held and used, fair value						0.0		0.0
Conesville [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets held and used, fair value						1.1		1.1
Conesville [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value						21.9		21.9
Fixed-asset impairment (Note 15)								20.8
Conesville [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets held and used, fair value						1.1		1.1
Hutchings Peakers [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value						3.2		3.2
Fixed-asset impairment (Note 15)								1.6
Hutchings Peakers [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets held and used, fair value						0.0		0.0
Hutchings Peakers [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets held and used, fair value						0.0		0.0
Hutchings Peakers [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets held and used, fair value						1.6		1.6
Hutchings Peakers [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value						3.0		3.0
Fixed-asset impairment (Note 15)								1.4
Hutchings Peakers [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets held and used, fair value						$ 1.6		1.6
Peaking Generating Facilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value							9.9
Fixed-asset impairment (Note 15)							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]
Long-lived assets held and used, fair value							0.0			0.0
Peaking Generating Facilities [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets held and used, fair value							0.0			0.0
Peaking Generating Facilities [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets held and used, fair value							$ 5.2
East Bend Station [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value										14.2
Long-lived assets held and used, fair value										2.7
Fixed-asset impairment (Note 15)					$ 11.5			$ 0.0		11.5
East Bend Station [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets held and used, fair value										$ 2.7

[1] Goodwill impairment of $135.8 million in 2014 has been reclassified to Discontinued operations in the Consolidated Statement of Operations.