Overview and Summary of Significant Accounting Policies (Narrative) (Details)		12 Months Ended
	Jan. 31, 2017 employee	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) mi² customer generating_facility segment	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Significant Accounting Policies [Line Items]
Capitalized Software, estimated amortization expense for year after next			$ 5,600,000
Capitalized Computer Software, Gross			$ 65,100,000	$ 59,900,000
Number of reportable segments | segment			2
Service area, square miles | mi²			6,000
Number of coal fired power plants | generating_facility			5
Capitalized interest for unregulated generation propety			$ 2,800,000	$ 2,000,000	$ 1,500,000
Straight-line depreciation average annual composite basis (percent)			6.10%	4.40%	5.30%
Depreciation and amortization			$ 132,300,000	$ 134,600,000	$ 135,600,000
Insurance and claims costs			5,400,000	5,900,000
Insurance costs below coverage thresholds of third-party providers			12,000,000	13,700,000
Investment in trust			300,000	300,000
Capitalized Computer Software, Accumulated Amortization			$ 43,200,000	35,300,000
Finite-Lived Intangible Asset, Useful Life			7 years
Capitalized Computer Software, Amortization			$ 7,700,000	9,000,000	8,600,000
Capitalized Software, estimated amortization over remaining useful life			15,300,000
Capitalized Software, estimated amortization expense for next twelve months			6,100,000
Capitalized Software, estimated amortization expense for three years in the future			4,000,000
Capitalized Software, estimated amortization expense for four years in the future			0
Capitalized Software, estimated amortization expense for five years in the future			0
THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Capitalized Software, estimated amortization expense for year after next			5,600,000
Capitalized Computer Software, Gross			$ 78,500,000	73,900,000
Number of reportable segments | segment			2
Approximate number of retail customers | customer			519,000
Service area, square miles | mi²			6,000
Number of coal fired power plants | generating_facility			5
Capitalized interest for unregulated generation propety			$ 2,700,000	$ 2,000,000	$ 1,500,000
Straight-line depreciation average annual composite basis (percent)			4.60%	2.50%	2.80%
Depreciation and amortization			$ 120,300,000	$ 138,200,000	$ 144,800,000
Insurance costs below coverage thresholds of third-party providers			11,800,000	13,700,000
Capitalized Computer Software, Accumulated Amortization			$ 56,400,000	49,200,000
Finite-Lived Intangible Asset, Useful Life			7 years
Capitalized Computer Software, Amortization			$ 7,500,000	8,200,000	8,000,000
Capitalized Software, estimated amortization over remaining useful life			15,300,000
Capitalized Software, estimated amortization expense for next twelve months			6,100,000
Capitalized Software, estimated amortization expense for three years in the future			4,000,000
Capitalized Software, estimated amortization expense for four years in the future			0
Capitalized Software, estimated amortization expense for five years in the future			0
Note to DPL Capital Trust II Maturing in 2031 - 8.125% [Member]
Significant Accounting Policies [Line Items]
Note payable to trust			15,600,000	15,600,000
Subsequent Event [Member]
Significant Accounting Policies [Line Items]
Entity number of employees | employee	1,168
Employees under collective bargaining agreement (percent)	62.00%
Subsequent Event [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Entity number of employees | employee	1,160
Percentage Of Employees Under Collective Bargaining Agreement	63.00%
Electric Generation, Transmission and Distribution Equipment [Member]
Significant Accounting Policies [Line Items]
Depreciation and amortization			121,900,000	125,900,000	128,100,000
Electric Generation, Transmission and Distribution Equipment [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Depreciation and amortization			110,000,000	132,700,000	141,600,000
Pension [Member]
Significant Accounting Policies [Line Items]
Service cost			5,700,000	7,100,000	5,900,000
Interest cost			14,700,000	17,300,000	17,500,000
Pension [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Service cost			5,700,000	7,100,000	5,900,000
Interest cost			14,700,000	17,300,000	$ 17,500,000
Pension [Member] | Scenario, Forecast [Member]
Significant Accounting Policies [Line Items]
Service cost		$ 5,700,000
Pension [Member] | Scenario, Forecast [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Service cost		$ 5,700,000
Disaggregated Rate Approach [Member]
Significant Accounting Policies [Line Items]
Service cost			5,900,000
Interest cost			15,300,000
Disaggregated Rate Approach [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Service cost			5,900,000
Interest cost			15,300,000
Disaggregated Rate Approach [Member] | Pension [Member]
Significant Accounting Policies [Line Items]
Service cost			5,700,000
Interest cost			14,700,000
Disaggregated Rate Approach [Member] | Pension [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Service cost			5,700,000
Interest cost			14,700,000
Disaggregated Rate Approach [Member] | Postretirement [Member]
Significant Accounting Policies [Line Items]
Service cost			200,000
Interest cost			600,000
Disaggregated Rate Approach [Member] | Postretirement [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Service cost			200,000
Interest cost			600,000
Aggregate Rate Approach [Member]
Significant Accounting Policies [Line Items]
Service cost			6,300,000
Interest cost			18,600,000
Aggregate Rate Approach [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Service cost			6,300,000
Interest cost			18,600,000
Aggregate Rate Approach [Member] | Pension [Member]
Significant Accounting Policies [Line Items]
Service cost			6,100,000
Interest cost			17,900,000
Aggregate Rate Approach [Member] | Pension [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Service cost			6,100,000
Interest cost			17,900,000
Aggregate Rate Approach [Member] | Postretirement [Member]
Significant Accounting Policies [Line Items]
Service cost			200,000
Interest cost			700,000
Aggregate Rate Approach [Member] | Postretirement [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Service cost			200,000
Interest cost			700,000
Impact of Change [Member]
Significant Accounting Policies [Line Items]
Service cost			(400,000)
Interest cost			(3,300,000)
Impact of Change [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Service cost			(400,000)
Interest cost			(3,300,000)
Impact of Change [Member] | Pension [Member]
Significant Accounting Policies [Line Items]
Service cost			(400,000)
Interest cost			(3,200,000)
Impact of Change [Member] | Pension [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Service cost			(400,000)
Interest cost			(3,200,000)
Impact of Change [Member] | Postretirement [Member]
Significant Accounting Policies [Line Items]
Service cost			0
Interest cost			(100,000)
Impact of Change [Member] | Postretirement [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Service cost			0
Interest cost			$ (100,000)
Adjustments for New Accounting Pronouncement [Member]
Significant Accounting Policies [Line Items]
Deferred Finance Costs, Current, Net				2,100,000
Deferred Finance Costs, Noncurrent, Net				14,000,000
Adjustments for New Accounting Pronouncement [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Deferred Finance Costs, Current, Net				1,800,000
Deferred Finance Costs, Noncurrent, Net				4,500,000
Line of Credit [Member]
Significant Accounting Policies [Line Items]
Deferred Finance Costs, Noncurrent, Net				3,100,000
Line of Credit [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Deferred Finance Costs, Noncurrent, Net				$ 700,000