Benefit Plans (Narrative) (Details) - USD ($)		1 Months Ended	12 Months Ended
	Jan. 31, 2017	Jan. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Amount Billed to Service Company				$ 1,300,000	$ 2,200,000
Defined contribution plan, maximum annual contributions per employee (percent)				85.00%
Employer contributions to defined contribution plan				$ 4,900,000	4,800,000	$ 4,700,000
Accumulated benefit obligation for our defined benefit pension plans				409,200,000	401,200,000
THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Amount Billed to Service Company				$ 1,300,000	2,200,000
Defined contribution plan, maximum annual contributions per employee (percent)				85.00%
Employer contributions to defined contribution plan				$ 4,900,000	4,800,000	4,700,000
Accumulated benefit obligation for our defined benefit pension plans				$ 409,200,000	401,200,000
Defined Benefit Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan employee vested percentage				100.00%
Defined benefit plan employee vested minimum period, years				5 years
Defined benefit plan, percent forfeited if terminated, other than by death or disability, prior to full vesting (percent)				100.00%
Defined Benefit Plan [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan employee vested percentage				100.00%
Defined benefit plan employee vested minimum period, years				5 years
Defined benefit plan, percent forfeited if terminated, other than by death or disability, prior to full vesting (percent)				100.00%
Cash Balance Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan employee vested percentage				100.00%
Defined benefit plan employee vested minimum period, years				3 years
Defined benefit plan, percent forfeited if terminated, other than by death or disability, prior to full vesting (percent)				100.00%
Management Employees [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan employee vested percentage				100.00%
Defined benefit plan employee vested minimum period, years				3 years
Defined benefit plan, percent forfeited if terminated, other than by death or disability, prior to full vesting (percent)				100.00%
Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension Contributions				$ 5,000,000	$ 5,000,000	$ 0
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets				6.50%	6.50%	6.75%
Discount rate for obligations				4.28%	4.49%	4.02%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate				4.49%	4.02%	4.86%
Defined Benefit Plan, Fair Value of Plan Assets				$ 341,000,000	$ 345,400,000	$ 371,700,000
Service cost				5,700,000	7,100,000	5,900,000
Interest cost				14,700,000	17,300,000	17,500,000
Defined Benefit Plan, Funded Status of Plan				$ (78,600,000)	(65,400,000)
Defined benefit plan, amortization period for underfunding excess				7 years
Pension [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension Contributions				$ 5,000,000	$ 5,000,000	$ 0
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets				6.50%	6.50%	6.75%
Discount rate for obligations				4.28%	4.49%	4.02%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate				4.49%	4.02%	4.86%
Defined Benefit Plan, Fair Value of Plan Assets				$ 341,000,000	$ 345,400,000	$ 371,700,000
Service cost				5,700,000	7,100,000	5,900,000
Interest cost				14,700,000	17,300,000	$ 17,500,000
Defined Benefit Plan, Funded Status of Plan				$ (78,600,000)	(65,400,000)
Defined benefit plan, amortization period for underfunding excess				7 years
Postretirement [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded Status of Plan				$ 15,800,000	15,000,000
Postretirement [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded Status of Plan				15,800,000	$ 15,000,000
SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated contribution to the defined benefit plans next year				$ 400,000
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target allocations for plan assets, fixed income securities, minimum				28.00%
Target allocations for plan assets, fixed income securities, maximum				48.00%
Equity Securities [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Target allocations for plan assets, fixed income securities, minimum				28.00%
Target allocations for plan assets, fixed income securities, maximum				48.00%
Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target allocations for plan assets, fixed income securities, minimum				42.00%
Target allocations for plan assets, fixed income securities, maximum				70.00%
Fixed Income Securities [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Target allocations for plan assets, fixed income securities, minimum				42.00%
Target allocations for plan assets, fixed income securities, maximum				70.00%
Scenario, Forecast [Member] | Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost			$ 5,700,000
Defined Benefit Plan, Administration Expenses			600,000
Scenario, Forecast [Member] | Pension [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost			5,700,000
Defined Benefit Plan, Administration Expenses			600,000
Increase in Expected Rate of Return [Member]
Defined Benefit Plan Disclosure [Line Items]
Change in Expected rate of return on plan assets				1.00%
Change in Pension Expense				$ (3,500,000)
Increase in Expected Rate of Return [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Change in Expected rate of return on plan assets				1.00%
Change in Pension Expense				$ (3,500,000)
Decrease in Expected Rate of Return [Member]
Defined Benefit Plan Disclosure [Line Items]
Change in Pension Expense				$ 3,500,000
Decrease in Expected Rate of Return [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Change in Expected rate of return on plan assets				1.00%
Change in Pension Expense				$ 3,500,000
Expected Increase in Discount Rate [Member]
Defined Benefit Plan Disclosure [Line Items]
Change in Pension Expense				(300,000)
Expected Increase in Discount Rate [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Change in Pension Expense				$ (300,000)
Change in Discount Rate				0.25%
Expected Decrease in Discount Rate [Member]
Defined Benefit Plan Disclosure [Line Items]
Change in Pension Expense				$ 400,000
Expected Decrease in Discount Rate [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Change in Pension Expense				$ 400,000
Change in Discount Rate				0.25%
Non-union Participant [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, period after which participant is fully vested in employer contributions				2 years
Non-union Participant [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, period after which participant is fully vested in employer contributions				2 years
Non-union Participant [Member] | First 1% of Eligible Compensation [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, employer matching contribution (percent)				100.00%
Defined contribution plan, employer matching contribution, percent of employees' gross pay (percent)				1.00%
Non-union Participant [Member] | First 1% of Eligible Compensation [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, employer matching contribution (percent)				100.00%
Defined contribution plan, employer matching contribution, percent of employees' gross pay (percent)				1.00%
Non-union Participant [Member] | Next 5% of Eligible Compensation [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, employer matching contribution (percent)				50.00%
Defined contribution plan, employer matching contribution, percent of employees' gross pay (percent)				5.00%
Non-union Participant [Member] | Next 5% of Eligible Compensation [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, employer matching contribution (percent)				50.00%
Defined contribution plan, employer matching contribution, percent of employees' gross pay (percent)				5.00%
Union Participant [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, employer matching contribution, percent of employees' gross pay (percent)				150.00%
Defined contribution plan, period after which participant is fully vested in employer contributions				3 years
Defined contribution plan, employer matching contribution cap				$ 2,200
Union Participant [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, employer matching contribution, percent of employees' gross pay (percent)				150.00%
Defined contribution plan, period after which participant is fully vested in employer contributions				3 years
Defined contribution plan, employer matching contribution cap				$ 2,200
Subsequent Event [Member] | Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension Contributions	$ 5,000,000
Subsequent Event [Member] | Pension [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension Contributions		$ 5,000,000
Subsequent Event [Member] | SERP [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated contribution to the defined benefit plans next year			$ 400,000