Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031897-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Name:	us-gaap_DebtInstrumentCarryingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Derivative Instruments and Hedging Activities (Narrative) (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($) Number_of_interest_rate_swaps	Dec. 31, 2018 USD ($)	Sep. 30, 2019 USD ($) Number_of_interest_rate_swaps	Mar. 29, 2018 USD ($)	Dec. 31, 2017 USD ($)
Sale of Derivative Instruments Interest Rate Swap		$ 60,000,000.0		$ 60,000,000.0
Gain on sale of Derivative Instruments Interest Rate Swap	$ 800,000	800,000
THE DAYTON POWER AND LIGHT COMPANY [Member]
Number of Interest Rate Swaps \| Number_of_interest_rate_swaps	2		2
Tax-exempt First Mortgage Bonds - rates from: 2.49% - 2.93% (a) and 1.29% - 1.42% (b)
Long-term Debt, Gross		140,000,000.0	$ 140,000,000.0		$ 200,000,000.0
Interest Rate Swap [Member] \| Designated as Hedging Instrument [Member]
Derivative, Notional Amount, Purchase (Sales), Net		140,000,000.0	$ 140,000,000.0		200,000,000.0
Sale of Derivative Instruments Interest Rate Swap		$ 0			$ 0