Debt (Long-term Debt Maturities) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Oct. 01, 2017
Debt Instrument [Line Items]
Unamortized Deferred Financing Costs, Consolidated	$ (10.6)
Current portion - long-term debt	103.6
Long-term Debt, Maturities, Repayments of Principal in Year Two	144.7
2018	784.7
2019	422.8
2020	0.2
Thereafter	32.4
Total Maturities	1,488.4
Total long-term debt	1,475.9
Disposal Group, Including Discontinued Operations, Long-Term Debt	0.0	$ 0.3
Unamortized Deferred Financing Costs	(4.3)	(6.8)
Current portion - long-term debt	103.6	4.6
Long-term Debt, Excluding Current Maturities	1,372.3	1,700.2
THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Unamortized Deferred Financing Costs, Consolidated	(6.3)
Current portion - long-term debt	4.6
Long-term Debt, Maturities, Repayments of Principal in Year Two	144.7
2018	4.7
2019	422.8
2020	0.2
Thereafter	16.8
Total Maturities	593.8
Debt Instrument, Unamortized Discount	(1.4)	(2.0)
Total long-term debt	586.1
Unamortized Deferred Financing Costs		(9.8)
Current portion - long-term debt	4.6	4.6
Long-term Debt, Excluding Current Maturities	581.5	642.0
Unamortized Deferred Financing Costs (Subsidiary)	(6.3)	(9.8)
Term Loan Maturing 2022 (DPL) [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	436.1	440.6
Term Loan Maturing 2022 (DPL) [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	436.1	440.6
One Point One Three To One Point One Seven Bonds Maturing In August Two Thousand Twenty [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	140.0	200.0
One Point One Three To One Point One Seven Bonds Maturing In August Two Thousand Twenty [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	140.0	200.0
Four Point Two Zero Percentage Of U S Government Note Maturing In February Two Thousand Sixty One [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	17.7	17.8
Four Point Two Zero Percentage Of U S Government Note Maturing In February Two Thousand Sixty One [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	17.7	17.8
Generation [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Disposal Group, Including Discontinued Operations, Long-Term Debt			$ 0.3
Debt [Member]
Debt Instrument [Line Items]
Long-term Debt and Capital Lease Obligations, Including Current Maturities	1,475.9	1,704.8
Debt [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Long-term Debt and Capital Lease Obligations, Including Current Maturities	$ 586.1	$ 646.6