Benefit Plans (Narrative) (Details) - USD ($)		12 Months Ended
	Jan. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Amount Billed to Service Company			$ 1,100,000	$ 1,300,000
Defined contribution plan, maximum annual contributions per employee (percent)			85.00%
Employer contributions to defined contribution plan			$ 3,100,000	5,100,000	$ 4,900,000
Accumulated benefit obligation for our defined benefit pension plans			428,300,000	409,200,000
THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Amount Billed to Service Company			$ 1,100,000	1,300,000
Defined contribution plan, maximum annual contributions per employee (percent)			85.00%
Employer contributions to defined contribution plan			$ 3,100,000	5,100,000	4,900,000
Accumulated benefit obligation for our defined benefit pension plans			428,300,000	409,200,000
Defined Benefit Plan, Amount Billed to AES Ohio Generation			$ 700,000
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%
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%
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%
Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Payment for Pension Benefits			$ 5,000,000	$ 5,000,000	$ 5,000,000
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets			6.50%	6.50%	6.50%
Discount rate for obligations			3.66%	4.28%	4.49%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate			4.28%	4.49%	4.02%
Defined Benefit Plan, Fair Value of Plan Assets			$ 357,500,000	$ 341,000,000	$ 345,400,000
Service cost			5,700,000	5,700,000	7,100,000
Interest cost			14,200,000	14,700,000	17,300,000
Defined Benefit Plan, Funded (Unfunded) Status of Plan			$ (79,400,000)	(78,600,000)
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]
Payment for Pension Benefits			$ 5,000,000	$ 5,000,000	$ 5,000,000
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets			6.50%	6.50%	6.50%
Discount rate for obligations			3.66%	4.28%	4.49%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate			4.28%	4.49%	4.02%
Defined Benefit Plan, Fair Value of Plan Assets			$ 357,500,000	$ 341,000,000	$ 345,400,000
Service cost			5,700,000	5,700,000	7,100,000
Interest cost			14,200,000	14,700,000	$ 17,300,000
Defined Benefit Plan, Funded (Unfunded) Status of Plan			$ (79,400,000)	(78,600,000)
Defined benefit plan, amortization period for underfunding excess			7 years
Postretirement [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded (Unfunded) Status of Plan			$ 12,700,000	15,800,000
Postretirement [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded (Unfunded) Status of Plan			$ 12,700,000	$ 15,800,000
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage			38.00%
Equity Securities [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage			38.00%
Scenario, Forecast [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase in pension cost due to change in return on assets		$ 3,400,000
Decrease in pension cost due to change in return on assets		(3,400,000)
Decrease in pension cost due to change in discount rate		(600,000)
Increase in pension cost due to change in discount rate		600,000
Scenario, Forecast [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase in pension cost due to change in return on assets		3,400,000
Decrease in pension cost due to change in return on assets		(3,400,000)
Decrease in pension cost due to change in discount rate		(600,000)
Increase in pension cost due to change in discount rate		600,000
Scenario, Forecast [Member] | Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		7,500,000
Defined Benefit Plan, Plan Assets, Administration Expense		2,200,000
Scenario, Forecast [Member] | Pension [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		7,500,000
Defined Benefit Plan, Plan Assets, Administration Expense		$ 2,200,000
Scenario, Forecast [Member] | SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Target Allocation Percentage		99.00%
Estimated contribution to the defined benefit plans next year		$ 400,000
Scenario, Forecast [Member] | SERP [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Target Allocation Percentage		99.00%
Estimated contribution to the defined benefit plans next year		$ 400,000
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			$ 2,300
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			$ 2,300
Subsequent Event [Member] | Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Payment for Pension Benefits	$ 7,500,000
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets		6.25%
Subsequent Event [Member] | Pension [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Payment for Pension Benefits	$ 7,500,000
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets		6.25%
Subsequent Event [Member] | Increase in Expected Rate of Return [Member]
Defined Benefit Plan Disclosure [Line Items]
Change in Expected rate of return on plan assets		1.00%
Subsequent Event [Member] | 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%
Subsequent Event [Member] | 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%
Subsequent Event [Member] | Expected Increase in Discount Rate [Member]
Defined Benefit Plan Disclosure [Line Items]
Change in discount rate for plan assets		25.00%
Subsequent Event [Member] | Expected Increase in Discount Rate [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Change in discount rate for plan assets		25.00%
Subsequent Event [Member] | Expected Decrease in Discount Rate [Member]
Defined Benefit Plan Disclosure [Line Items]
Change in discount rate for plan assets		25.00%
Subsequent Event [Member] | Expected Decrease in Discount Rate [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Change in discount rate for plan assets		25.00%
Minimum [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage			24.00%
Minimum [Member] | Equity Securities [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage			24.00%
Minimum [Member] | Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage			47.00%
Minimum [Member] | Fixed Income Securities [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage			47.00%
Maximum [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage			52.00%
Maximum [Member] | Equity Securities [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage			52.00%
Maximum [Member] | Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage			65.00%
Maximum [Member] | Fixed Income Securities [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage			65.00%