Business Segments (Narrative) (Details) - THE DAYTON POWER AND LIGHT COMPANY [Member]	9 Months Ended
Sep. 30, 2018 mi² customer segment
Segment Reporting Information [Line Items]
Number of Operating Segments \| segment	1
Approximate number of retail customers \| customer	524,000
Service area, square miles \| mi²	6,000

Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues.

No definition available.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration