Business Segments (Segment Financial Information) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
External customer revenues	$ 775.9	$ 743.9	$ 834.2
Intersegment revenues	0.0	0.0	0.0
Total revenues	775.9	743.9	834.2
Fuel Costs	17.5	9.0	17.4
Depreciation and amortization	73.1	76.1	73.6
Interest expense	98.0	110.0	107.4
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	31.9	(6.5)	12.4
Net loss from continuing operations	31.2	(1.5)	14.8
Discontinued operations, net of tax	38.9	(93.1)	(500.0)
Net income (loss)	70.1	(94.6)	(485.2)
Cash capital expenditures	103.6	121.5	148.5
Total assets (end of year)	1,883.1	2,049.2	2,419.2
Fixed-asset impairment (Note 14)	2.8	0.0	23.9
Operating Segments [Member] | Transmission and Distribution [Member]
Segment Reporting Information [Line Items]
External customer revenues	737.8	718.9	806.7
Intersegment revenues	0.9	1.1	1.3
Total revenues	738.7	720.0	808.0
Depreciation and amortization	74.5	75.3	71.0
Interest expense	27.3	30.5	25.4
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	104.4	88.5	143.0
Cash capital expenditures	93.1	85.6	83.4
Total assets (end of year)	1,819.6	1,695.9	1,710.5
Fixed-asset impairment (Note 14)	0.0		0.0
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
External customer revenues	38.1	25.0	27.5
Intersegment revenues	2.9	4.4	5.7
Total revenues	41.0	29.4	33.2
Depreciation and amortization	(1.4)	0.8	2.6
Interest expense	70.7	79.5	82.3
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	(72.5)	(95.0)	(130.6)
Cash capital expenditures	10.5	35.9	65.1
Total assets (end of year)	545.9	736.5	1,145.9
Fixed-asset impairment (Note 14)	2.8		23.9
Consolidation, Eliminations [Member]
Segment Reporting Information [Line Items]
External customer revenues	0.0	0.0	0.0
Intersegment revenues	(3.8)	(5.5)	(7.0)
Total revenues	(3.8)	(5.5)	(7.0)
Depreciation and amortization	0.0	0.0	0.0
Interest expense	0.0	0.0	(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)	(482.4)	(383.2)	(437.2)
Fixed-asset impairment (Note 14)	0.0		0.0
THE DAYTON POWER AND LIGHT COMPANY [Member]
Segment Reporting Information [Line Items]
Fuel Costs	2.4	0.5	5.3
Depreciation and amortization	74.5	75.3	71.0
Interest expense	27.3	30.5	24.7
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	104.4	88.5	143.6
Net loss from continuing operations	86.7	57.4	97.6
Discontinued operations, net of tax	0.0	(40.4)	(870.3)
Net income (loss)	86.7	17.0	(772.7)
Total assets (end of year)	1,819.6	1,695.9
Fixed-asset impairment (Note 14)	$ 0.0	66.3	1,353.5
THE DAYTON POWER AND LIGHT COMPANY [Member] | Generation [Member]
Segment Reporting Information [Line Items]
Discontinued operations, net of tax		$ (40.4)	$ (870.3)