Overview and Summary of Significant Accounting Policies (Narrative) (Details) $ in Millions			12 Months Ended
	Jan. 31, 2019 employee	Jan. 01, 2018 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) mi² segment customer	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Significant Accounting Policies [Line Items]
Other Investments				$ 0.2	$ 0.3
Cash and Cash Equivalents, at Carrying Value				90.5	24.5
Restricted Cash and Cash Equivalents, Current				21.2	0.4
Capitalized Software, estimated amortization expense for year after next				$ 3.2
Number of reportable segments | segment				1
Service area, square miles | mi²				6,000
Capitalized interest for unregulated generation property				$ 0.5	$ 1.7	$ 2.1
Straight-line depreciation average annual composite basis (percent)				4.30%	5.00%	6.10%
Depreciation and amortization				$ 73.1	$ 76.1	$ 73.6
Insurance and claims costs				4.1	3.0
Insurance costs below coverage thresholds of third-party providers				$ 4.3	4.4
Finite-Lived Intangible Asset, Useful Life				7 years
Capitalized Computer Software, Amortization				$ 6.6	5.7	6.6
Capitalized Software, estimated amortization over remaining useful life				15.0
Capitalized Software, estimated amortization expense for next twelve months				4.2
Capitalized Software, estimated amortization expense for three years in the future				3.0
Capitalized Software, estimated amortization expense for four years in the future				2.6
Capitalized Software, estimated amortization expense for five years in the future				2.0
AOCI reclassed to Retained Earnings before tax		$ 1.6
AOCI reclassed to Retained Earnings, net of tax		1.0		(1.0)
Restricted Cash and Cash Equivalents				111.7	24.9	54.6	$ 80.3
THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Cash and Cash Equivalents, at Carrying Value				45.0	5.2
Restricted Cash and Cash Equivalents, Current				21.2	0.4
Payments of Ordinary Dividends, Common Stock				43.8	39.0	70.0
Capitalized Software, estimated amortization expense for year after next				$ 2.4
Approximate number of retail customers | customer				525,000
Service area, square miles | mi²				6,000
Capitalized interest for unregulated generation property				$ 0.5	$ 1.5	$ 2.0
Straight-line depreciation average annual composite basis (percent)				3.00%	3.40%	4.60%
Depreciation and amortization				$ 74.5	$ 75.3	$ 71.0
Insurance costs below coverage thresholds of third-party providers				$ 4.3	4.4
Finite-Lived Intangible Asset, Useful Life				7 years
Capitalized Computer Software, Amortization				$ 6.3	5.7	6.7
Capitalized Software, estimated amortization over remaining useful life				11.1
Capitalized Software, estimated amortization expense for next twelve months				3.5
Capitalized Software, estimated amortization expense for three years in the future				2.2
Capitalized Software, estimated amortization expense for four years in the future				1.8
Capitalized Software, estimated amortization expense for five years in the future				1.2
AOCI reclassed to Retained Earnings before tax		1.7
AOCI reclassed to Retained Earnings, net of tax		$ 1.1		(1.1)
Restricted Cash and Cash Equivalents				66.2	5.6	1.6	$ 5.4
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	659
Employees under collective bargaining agreement (percent)	57.00%
Subsequent Event [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Entity number of employees | employee	647
Percentage Of Employees Under Collective Bargaining Agreement	58.00%
Electric Generation, Transmission and Distribution Equipment [Member]
Significant Accounting Policies [Line Items]
Depreciation and amortization				66.5	70.4	67.0
Electric Generation, Transmission and Distribution Equipment [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Depreciation and amortization				68.2	69.6	64.3
Pension [Member]
Significant Accounting Policies [Line Items]
Service cost				6.1	5.7	5.7
Interest cost				13.8	14.2	14.7
Pension [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Service cost				6.1	5.7	5.7
Interest cost				13.8	14.2	14.7
Pension [Member] | Scenario, Forecast [Member]
Significant Accounting Policies [Line Items]
Service cost			$ 5.4
Pension [Member] | Scenario, Forecast [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Service cost			$ 5.4
Adjustments for New Accounting Pronouncement [Member]
Significant Accounting Policies [Line Items]
Non-service Pension Costs					2.2	3.2
Increase (Decrease) in Restricted Cash					27.1	(11.8)
AOCI reclassed to Retained Earnings before tax				1.6
AOCI reclassed to Retained Earnings, net of tax				1.0
Adjustments for New Accounting Pronouncement [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Non-service Pension Costs					(1.5)	(0.9)
Increase (Decrease) in Restricted Cash					$ 26.6	$ (11.9)
AOCI reclassed to Retained Earnings before tax				1.7
AOCI reclassed to Retained Earnings, net of tax				$ 1.1