Overview and Summary of Significant Accounting Policies (Narrative) (Details) $ in Millions		9 Months Ended	12 Months Ended
	Jan. 31, 2018 employee	Sep. 30, 2017 generating_facility	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) mi² customer generating_facility segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Significant Accounting Policies [Line Items]
Capitalized Software, estimated amortization expense for year after next				$ 4.2
Number of reportable segments | segment				2
Service area, square miles | mi²				6,000
Number of coal fired power plants | generating_facility		5		3
Capitalized interest for unregulated generation property				$ 2.3	$ 2.8	$ 2.0
Straight-line depreciation average annual composite basis (percent)				5.00%	6.10%	4.40%
Depreciation and amortization				$ 106.9	$ 132.3	$ 134.6
Insurance and claims costs				3.0	5.4
Insurance costs below coverage thresholds of third-party providers				11.9	10.9
Investment in trust				$ 0.3	0.3
Finite-Lived Intangible Asset, Useful Life				7 years
Capitalized Computer Software, Amortization				$ 6.8	7.7	9.0
Capitalized Software, estimated amortization over remaining useful life				17.2
Capitalized Software, estimated amortization expense for next twelve months				7.2
Capitalized Software, estimated amortization expense for three years in the future				2.7
Capitalized Software, estimated amortization expense for four years in the future				1.7
Capitalized Software, estimated amortization expense for five years in the future				1.4
Increase (Decrease) in Restricted Cash				(27.1)	11.8	0.4
THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Payments of Ordinary Dividends, Common Stock				39.0	70.0	50.0
Capitalized Software, estimated amortization expense for year after next				$ 3.1
Approximate number of retail customers | customer				521,000
Service area, square miles | mi²				6,000
Number of coal fired power plants | generating_facility				5
Capitalized interest for unregulated generation property				$ 2.0	$ 2.7	$ 2.0
Straight-line depreciation average annual composite basis (percent)				3.40%	4.60%	2.50%
Depreciation and amortization				$ 75.3	$ 71.0	$ 71.5
Insurance costs below coverage thresholds of third-party providers				$ 4.4	3.9
Finite-Lived Intangible Asset, Useful Life				7 years
Capitalized Computer Software, Amortization				$ 5.7	6.7	7.2
Capitalized Software, estimated amortization over remaining useful life				10.7
Capitalized Software, estimated amortization expense for next twelve months				5.1
Capitalized Software, estimated amortization expense for three years in the future				1.6
Capitalized Software, estimated amortization expense for four years in the future				0.6
Capitalized Software, estimated amortization expense for five years in the future				0.3
Increase (Decrease) in Restricted Cash				(26.6)	11.9	0.3
Note to DPL Capital Trust II Maturing in 2031 - 8.125% [Member]
Significant Accounting Policies [Line Items]
Note payable to trust				15.6	15.6
Subsequent Event [Member]
Significant Accounting Policies [Line Items]
Entity number of employees | employee	1,060
Employees under collective bargaining agreement (percent)	60.00%
Subsequent Event [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Entity number of employees | employee	660
Percentage Of Employees Under Collective Bargaining Agreement	53.00%
Electric Generation, Transmission and Distribution Equipment [Member]
Significant Accounting Policies [Line Items]
Depreciation and amortization				100.1	124.6	125.6
Electric Generation, Transmission and Distribution Equipment [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Depreciation and amortization				69.6	64.3	64.3
Pension [Member]
Significant Accounting Policies [Line Items]
Service cost				5.7	5.7	7.1
Interest cost				14.2	14.7	17.3
Pension [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Service cost				5.7	5.7	7.1
Interest cost				14.2	14.7	$ 17.3
Pension [Member] | Scenario, Forecast [Member]
Significant Accounting Policies [Line Items]
Service cost			$ 7.5
Pension [Member] | Scenario, Forecast [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Service cost			$ 7.5
Adjustments for New Accounting Pronouncement [Member]
Significant Accounting Policies [Line Items]
Non-service Pension Costs				1.9	1.8
Increase (Decrease) in Restricted Cash				27.1	(11.8)
Adjustments for New Accounting Pronouncement [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Non-service Pension Costs				7.2	7.8
Increase (Decrease) in Restricted Cash				$ 26.6	$ (11.9)