Business Segments (Segment Financial Information) (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
External customer revenues		$ 1,236.9	$ 1,427.3	$ 1,612.8
Intersegment revenues		0.0	0.0	0.0
Total revenues		1,236.9	1,427.3	1,612.8
Fuel Costs		210.3	268.8	259.8
Depreciation and amortization		106.9	132.3	134.6
Goodwill, Impairment Loss, Excluding Discontinued Operation		0.0	0.0	317.0
Interest expense		110.1	107.7	119.8
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest		(119.9)	(793.3)	(231.4)
Net loss from continuing operations		(94.6)	(514.5)	(251.4)
Discontinued operations, net of tax		0.0	29.3	12.4
Net income (loss)		(94.6)	(485.2)	(239.0)
Cash capital expenditures		121.5	148.5	137.2
Total assets (end of year) (a)	$ 2,419.2	2,049.2	2,419.2	3,324.7
Fixed-asset impairment (Note 15)	623.5	175.8	859.0	0.0
Operating Segments [Member] | Transmission and Distribution [Member]
Segment Reporting Information [Line Items]
OVEC Revenue				19.7
External customer revenues		718.9	806.7	855.5
Intersegment revenues		1.1	1.3	1.5
Total revenues		720.0	808.0	857.0
Depreciation and amortization		75.3	71.0	71.5
Goodwill, Impairment Loss, Excluding Discontinued Operation				0.0
Interest expense		30.5	25.4	29.8
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest		88.5	143.0	188.1
Cash capital expenditures		85.6	83.4	98.3
Total assets (end of year) (a)	1,710.5	1,689.4	1,710.5	1,688.8
Fixed-asset impairment (Note 15)		0.0	0.0
Operating Segments [Member] | Generation [Member]
Segment Reporting Information [Line Items]
External customer revenues		507.9	611.5	770.3
Intersegment revenues		0.0	0.0	186.6
Total revenues		507.9	611.5	956.9
Depreciation and amortization		20.9	55.4	72.6
Goodwill, Impairment Loss, Excluding Discontinued Operation				0.0
Interest expense		0.1	0.4	2.9
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest		(18.5)	(1,353.9)	(28.7)
Cash capital expenditures		31.3	64.2	35.2
Total assets (end of year) (a)	472.3	275.0	472.3	1,805.0
Fixed-asset impairment (Note 15)		66.3	1,353.5
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
External customer revenues		10.1	9.1	6.7
Intersegment revenues		4.4	5.7	4.2
Total revenues		14.5	14.8	10.9
Depreciation and amortization		10.7	5.9	(9.5)
Goodwill, Impairment Loss, Excluding Discontinued Operation				317.0
Interest expense		79.5	82.2	87.4
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest		(189.9)	417.6	(390.8)
Cash capital expenditures		4.6	0.9	3.7
Total assets (end of year) (a)	673.6	468.0	673.6	1,170.3
Fixed-asset impairment (Note 15)		109.5	(494.5)
Consolidation, Eliminations [Member]
Segment Reporting Information [Line Items]
External customer revenues		0.0	0.0	(19.7)
Intersegment revenues		(5.5)	(7.0)	(192.3)
Total revenues		(5.5)	(7.0)	(212.0)
Depreciation and amortization		0.0	0.0	0.0
Goodwill, Impairment Loss, Excluding Discontinued Operation				0.0
Interest expense		0.0	(0.3)	(0.3)
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest		0.0	0.0	0.0
Cash capital expenditures		0.0	0.0	0.0
Total assets (end of year) (a)	(437.2)	(383.2)	(437.2)	(1,339.4)
Fixed-asset impairment (Note 15)		0.0	0.0
THE DAYTON POWER AND LIGHT COMPANY [Member]
Segment Reporting Information [Line Items]
Fuel Costs		0.5	5.3	(9.0)
Depreciation and amortization		75.3	71.0	71.5
Interest expense		30.5	24.7	28.9
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest		88.5	143.6	189.0
Net loss from continuing operations		57.4	97.6	130.0
Discontinued operations, net of tax		(40.4)	(870.3)	(23.6)
Net income (loss)		17.0	(772.7)	106.4
Total assets (end of year) (a)	$ 2,035.1	1,689.4	2,035.1
Fixed-asset impairment (Note 15)		66.3	1,353.5	0.0
THE DAYTON POWER AND LIGHT COMPANY [Member] | Generation [Member]
Segment Reporting Information [Line Items]
Discontinued operations, net of tax		$ (40.4)	$ (870.3)	$ (23.6)