Regulatory Assets and Liabilities (Schedule of Regulatory Assets and Liabilities) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Total current regulatory assets	$ 23.9	$ 0.1
Total non-current regulatory assets	163.2	203.9
Total regulatory assets	187.1	204.0
Total current regulatory liabilities	14.8	33.7
Total non-current regulatory liabilities	221.2	130.4
Total regulatory liabilities	$ 236.0	164.1
Overcollection of costs to be refunded [Member]
Regulatory Liabilities Type of Recovery	A/B
Total non-current regulatory liabilities	$ 14.8	33.7
Regulatory Liability, Amortization Through	2018
Postretirement benefits [Member]
Regulatory Liabilities Type of Recovery	B
Total non-current regulatory liabilities	$ 5.0	3.9
Regulatory Liability, Amortization Through	Ongoing
Removal Costs [Member]
Total non-current regulatory liabilities	$ 132.8	126.5
Regulatory Liability, Amortization Through	Not Applicable
Deferred Income Tax Charge [Member]
Total non-current regulatory liabilities	$ 83.4	0.0
Regulatory Liability, Amortization Through	Various
THE DAYTON POWER AND LIGHT COMPANY [Member]
Total current regulatory assets	$ 23.9	0.1
Total non-current regulatory assets	163.2	203.9
Total regulatory assets	187.1	204.0
Total current regulatory liabilities	14.8	33.7
Total non-current regulatory liabilities	221.2	130.4
Total regulatory liabilities	$ 236.0	164.1
THE DAYTON POWER AND LIGHT COMPANY [Member] | Overcollection of costs to be refunded [Member]
Regulatory Liabilities Type of Recovery	A/B
Total non-current regulatory liabilities	$ 14.8	33.7
Regulatory Liability, Amortization Through	2018
THE DAYTON POWER AND LIGHT COMPANY [Member] | Postretirement benefits [Member]
Regulatory Liabilities Type of Recovery	B
Total non-current regulatory liabilities	$ 5.0	3.9
Regulatory Liability, Amortization Through	Ongoing
THE DAYTON POWER AND LIGHT COMPANY [Member] | Removal Costs [Member]
Total non-current regulatory liabilities	$ 132.8	126.5
Regulatory Liability, Amortization Through	Not Applicable
THE DAYTON POWER AND LIGHT COMPANY [Member] | Deferred Income Tax Charge [Member]
Total non-current regulatory liabilities	$ 83.4	0.0
Regulatory Liability, Amortization Through	Various
Undercollections to be collected [Member]
Type of Recovery	A/B
Amortization Through	2018
Total current regulatory assets	$ 23.9	0.1
Undercollections to be collected [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Type of Recovery	A/B
Amortization Through	2018
Total current regulatory assets	$ 23.9	0.1
Pension benefits [Member]
Type of Recovery	B
Amortization Through	Ongoing
Total non-current regulatory assets	$ 92.4	97.6
Pension benefits [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Type of Recovery	B
Amortization Through	Ongoing
Total non-current regulatory assets	$ 92.4	97.6
Deferred Income Tax Charge [Member]
Type of Recovery	B/C
Amortization Through	Ongoing
Total non-current regulatory assets	$ 0.0	35.9
Deferred Income Tax Charge [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Type of Recovery	B/C
Amortization Through	Ongoing
Total non-current regulatory assets	$ 0.0	35.9
Fuel costs [Member]
Type of Recovery	B
Amortization Through	2020
Total non-current regulatory assets	$ 9.3	15.4
Fuel costs [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Type of Recovery	B
Amortization Through	2020
Total non-current regulatory assets	$ 9.3	15.4
Deferred Regulatory Compliance Costs [Member]
Type of Recovery	B
Amortization Through	2020
Total non-current regulatory assets	$ 9.2	12.4
Deferred Regulatory Compliance Costs [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Type of Recovery	B
Amortization Through	2020
Total non-current regulatory assets	$ 9.2	12.4
Unrecovered OVEC charges [Member]
Type of Recovery	D
Amortization Through	Undetermined
Total non-current regulatory assets	$ 27.8	21.0
Unrecovered OVEC charges [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Type of Recovery	D
Amortization Through	Undetermined
Total non-current regulatory assets	$ 27.8	21.0
Unamortized loss on reacquired debt [Member]
Type of Recovery	B
Amortization Through	Various
Total non-current regulatory assets	$ 7.0	8.0
Unamortized loss on reacquired debt [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Type of Recovery	B
Amortization Through	Various
Total non-current regulatory assets	$ 7.0	8.0
Smart grid and advanced metering infrastructure costs [Member]
Type of Recovery	B
Amortization Through	Undetermined
Total non-current regulatory assets	$ 7.3	7.3
Smart grid and advanced metering infrastructure costs [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Type of Recovery	B
Amortization Through	Undetermined
Total non-current regulatory assets	$ 7.3	7.3
Rate case costs [Member]
Type of Recovery	B
Amortization Through	Undetermined
Total non-current regulatory assets	$ 8.1	6.3
Rate case costs [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Type of Recovery	B
Amortization Through	Undetermined
Total non-current regulatory assets	$ 8.1	6.3
Storm Costs [Member]
Type of Recovery	A
Amortization Through	Undetermined
Total non-current regulatory assets	$ 2.1	0.0
Storm Costs [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Type of Recovery	A
Amortization Through	Undetermined
Total non-current regulatory assets	$ 2.1	$ 0.0