Overview and Summary of Significant Accounting Policies (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2021 USD ($) mi² customer segment	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Aug. 01, 2018 USD ($)
Significant Accounting Policies [Line Items]
Other Investments	$ 0.2	$ 0.2
Cash and Cash Equivalents, at Carrying Value	26.6	25.4
Restricted Cash and Cash Equivalents, Current	0.1	0.1
Capitalized Software, estimated amortization expense for year after next	$ 2.1
Number of reportable segments | segment	1
Service area, square miles | mi²	6,000
Capitalized interest for unregulated generation property	$ 3.7	$ 3.0	$ 3.2
Straight-line depreciation average annual composite basis (percent)	3.70%	3.80%	4.00%
Depreciation and amortization	$ 76.1	$ 73.3	$ 72.3
Insurance and claims costs	2.3	3.2
Insurance costs below coverage thresholds of third-party providers	9.7	11.1
Capitalized Computer Software, Amortization	3.2	3.3	4.4
Capitalized Software, estimated amortization over remaining useful life	9.9
Capitalized Software, estimated amortization expense for next twelve months	2.8
Capitalized Software, estimated amortization expense for three years in the future	1.9
Capitalized Software, estimated amortization expense for four years in the future	1.6
Capitalized Software, estimated amortization expense for five years in the future	1.5
Restricted Cash and Cash Equivalents	26.7	25.5	47.0	$ 111.7
Carrying Value	1,700.8	1,565.3
Other Assets, Noncurrent	30.1	24.6
Customer receivables	42.3	48.5
Unbilled revenue	19.2	21.6
Amounts due from partners in jointly owned stations	3.1	0.2
Due from PJM transmission settlement	1.7	1.7			$ 40.6
Other	5.5	0.5
Accounts Receivable, Allowance for Credit Loss	(0.3)	(2.8)
Accounts receivable, net	71.5	69.7
Accounts Receivable, Allowance for Credit Loss, Current	0.3	2.8	0.4
Accounts Receivable, Credit Loss Expense (Reversal)	(0.4)	3.0
Accounts Receivable, Allowance for Credit Loss, Writeoff	(3.5)	(2.3)
Accounts Receivable, Allowance for Credit Loss, Recovery	1.4	1.7
Accumulated other comprehensive income/(loss)	(4.8)	(12.3)	(3.6)
Other comprehensive loss before reclassifications	6.4	(8.8)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	1.1	0.1
Other Comprehensive Income (Loss), Net of Tax	7.5	(8.7)	(5.8)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	(0.8)	(0.9)	(1.1)
Other Comprehensive Income (Loss), Defined Benefit Plan, Transition Asset (Obligation), Reclassification Adjustment from AOCI, after Tax	1.9	1.0	0.2
Interest expense	62.9	71.3	82.2
Income Tax Expense (Benefit)	(0.5)	(5.5)	(20.3)
Income tax expense from discontinued operations	(0.2)	0.1	14.1
Other income	(62.9)	(101.0)	(123.4)
Prepaid Implementation Costs for Software as a Service
Significant Accounting Policies [Line Items]
Other Assets, Noncurrent	8.0	4.1
THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Cash and Cash Equivalents, at Carrying Value	14.4	11.7
Restricted Cash and Cash Equivalents, Current	0.1	0.1
Payments of Ordinary Dividends, Common Stock	52.0		95.0
Capitalized Software, estimated amortization expense for year after next	$ 2.1
Approximate number of retail customers | customer	534,000
Service area, square miles | mi²	6,000
Capitalized interest for unregulated generation property	$ 3.7	$ 3.0	$ 3.2
Straight-line depreciation average annual composite basis (percent)	2.80%	2.80%	2.90%
Depreciation and amortization	$ 74.6	$ 71.8	$ 70.8
Insurance costs below coverage thresholds of third-party providers	3.4	3.5
Capitalized Software, estimated amortization over remaining useful life		9.9
Capitalized Software, estimated amortization expense for next twelve months	2.8
Capitalized Software, estimated amortization expense for three years in the future	1.9
Capitalized Software, estimated amortization expense for four years in the future	1.6
Capitalized Software, estimated amortization expense for five years in the future	1.5
Restricted Cash and Cash Equivalents	14.5	11.8	21.3	$ 66.2
Carrying Value	1,679.2	1,544.0
Other Assets, Noncurrent	29.9	24.0
Customer receivables	41.6	47.6
Unbilled revenue	19.2	21.6
Amounts due from partners in jointly owned stations	4.4	1.9
Due from PJM transmission settlement	1.7	1.7			$ 40.6
Other	5.3	0.2
Accounts Receivable, Allowance for Credit Loss	(0.3)	(2.8)
Accounts receivable, net	71.9	70.2
Accounts Receivable, Allowance for Credit Loss, Current	0.3	2.8	0.4
Accounts Receivable, Credit Loss Expense (Reversal)	(0.4)	3.0
Accounts Receivable, Allowance for Credit Loss, Writeoff	(3.5)	(2.3)
Accounts Receivable, Allowance for Credit Loss, Recovery	1.4	1.7
Accumulated other comprehensive income/(loss)	(31.8)	(42.1)	(36.9)
Other comprehensive loss before reclassifications		(8.9)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	3.9	3.7
Other Comprehensive Income (Loss), Net of Tax	10.3	(5.2)	(1.6)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	0.0	0.6	(0.2)
Other Comprehensive Income (Loss), Defined Benefit Plan, Transition Asset (Obligation), Reclassification Adjustment from AOCI, after Tax	3.9	3.1	3.0
Interest expense	24.2	24.3	26.0
Income Tax Expense (Benefit)	5.7	7.0	(0.6)
Other income	(26.7)	(25.4)	(26.4)
THE DAYTON POWER AND LIGHT COMPANY [Member] | Prepaid Implementation Costs for Software as a Service
Significant Accounting Policies [Line Items]
Other Assets, Noncurrent	8.0	4.1
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
Electric Generation, Transmission and Distribution Equipment [Member]
Significant Accounting Policies [Line Items]
Depreciation and amortization	72.9	70.0	67.9
Electric Generation, Transmission and Distribution Equipment [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Significant Accounting Policies [Line Items]
Depreciation and amortization	71.6	68.6	66.5
Reclassification out of Accumulated Other Comprehensive Income [Member]
Net income (loss)	1.1	0.1	(1.3)
Reclassification out of Accumulated Other Comprehensive Income [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Net income (loss)	3.9	3.7	2.8
Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss) Attributable to Parent | THE DAYTON POWER AND LIGHT COMPANY [Member]
Accumulated other comprehensive income/(loss)			(0.4)
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Accumulated other comprehensive income/(loss)	(17.6)	(25.9)	(18.1)
Other comprehensive loss before reclassifications	6.4	(8.8)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent		1.0
Other Comprehensive Income (Loss), Net of Tax	8.3	(7.8)
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Accumulated other comprehensive income/(loss)	(31.8)	(42.1)	(36.5)
Other comprehensive loss before reclassifications	6.4	(8.7)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent		3.1
Other Comprehensive Income (Loss), Net of Tax	10.3	(5.6)
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Income Tax Expense (Benefit)	0.6	0.3	0.0
Net income (loss)	1.9	1.0	0.2
Other income	2.5	1.3	0.2
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Income Tax Expense (Benefit)	1.1	0.9	0.5
Net income (loss)	3.9	3.1	3.0
Other income	5.0	4.0	3.5
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Accumulated other comprehensive income/(loss)	12.8	13.6	14.5
Other comprehensive loss before reclassifications	0.0	0.0
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent		(0.9)
Other Comprehensive Income (Loss), Net of Tax	(0.8)	(0.9)
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Accumulated other comprehensive income/(loss)	0.0	0.0
Other comprehensive loss before reclassifications	0.0	(0.2)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent		0.6
Other Comprehensive Income (Loss), Net of Tax	0.0	0.4
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Interest expense	1.0	1.1	1.2
Income Tax Expense (Benefit)	(0.2)	(0.2)	(0.1)
Net income (loss)	(0.8)	(0.9)	(1.1)
Income tax expense from discontinued operations	0.0	0.0	0.4
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Interest expense	0.0	0.2	0.2
Income Tax Expense (Benefit)	(0.0)	(0.8)	0.0
Net income (loss)	$ 0.0	$ 0.6	$ (0.2)